Fix the padding of draft comment textarea

Since release 1124 the padding in draft comments has become too narrow:
https://imgur.com/a/hOx8m8d

The padding in `iron-autogrow-textarea` is based on a css mixin. Css
mixins are legacy technology that we are trying to avoid by all means.
They need polyfills and special care. We use them only when we must,
which is for iron- and paper-components.

You can only use a css mixin, if it used in a *Polymer* element
somewhere. `gr-css-mixins` was introduced to take care of that. Maybe
`--iron-autogrow-textarea` was overlooked when it was created? Anyway,
adding it there solves the issue.

The root cause for the regression was probably that the shared styles
were referenced in `gr-overlay`, which was still a Polymer element.
The issue appeared as soon as the last Polymer element referencing the
shared styles was gone.

Release-Notes: skip
Change-Id: I0466417f12b2626778c2f7c278501f052c02c361
diff --git a/polygerrit-ui/app/elements/admin/gr-group-members/gr-group-members.ts b/polygerrit-ui/app/elements/admin/gr-group-members/gr-group-members.ts
index 87761a6..8824009 100644
--- a/polygerrit-ui/app/elements/admin/gr-group-members/gr-group-members.ts
+++ b/polygerrit-ui/app/elements/admin/gr-group-members/gr-group-members.ts
@@ -3,7 +3,6 @@
  * Copyright 2017 Google LLC
  * SPDX-License-Identifier: Apache-2.0
  */
-import '@polymer/iron-autogrow-textarea/iron-autogrow-textarea';
 import '../../shared/gr-account-label/gr-account-label';
 import '../../shared/gr-autocomplete/gr-autocomplete';
 import '../../shared/gr-button/gr-button';
diff --git a/polygerrit-ui/app/elements/admin/gr-repo-commands/gr-repo-commands.ts b/polygerrit-ui/app/elements/admin/gr-repo-commands/gr-repo-commands.ts
index 2a648b1..bb1e926 100644
--- a/polygerrit-ui/app/elements/admin/gr-repo-commands/gr-repo-commands.ts
+++ b/polygerrit-ui/app/elements/admin/gr-repo-commands/gr-repo-commands.ts
@@ -3,7 +3,6 @@
  * Copyright 2017 Google LLC
  * SPDX-License-Identifier: Apache-2.0
  */
-import '@polymer/iron-autogrow-textarea/iron-autogrow-textarea';
 import '../../plugins/gr-endpoint-decorator/gr-endpoint-decorator';
 import '../../plugins/gr-endpoint-param/gr-endpoint-param';
 import '../../shared/gr-button/gr-button';
diff --git a/polygerrit-ui/app/elements/gr-css-mixins.ts b/polygerrit-ui/app/elements/gr-css-mixins.ts
index 733b386..523a056 100644
--- a/polygerrit-ui/app/elements/gr-css-mixins.ts
+++ b/polygerrit-ui/app/elements/gr-css-mixins.ts
@@ -77,6 +77,10 @@
           --paper-listbox: {
             padding: 0;
           };
+          --iron-autogrow-textarea: {
+            box-sizing: border-box;
+            padding: var(--spacing-s);
+          };
         }
       </style>
     `;
diff --git a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
index d9004ae..5306cea 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
+++ b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
@@ -3,7 +3,6 @@
  * Copyright 2015 Google LLC
  * SPDX-License-Identifier: Apache-2.0
  */
-import '@polymer/iron-autogrow-textarea/iron-autogrow-textarea';
 import '../../../styles/shared-styles';
 import '../../plugins/gr-endpoint-decorator/gr-endpoint-decorator';
 import '../../plugins/gr-endpoint-param/gr-endpoint-param';
diff --git a/polygerrit-ui/app/styles/shared-styles.ts b/polygerrit-ui/app/styles/shared-styles.ts
index 839f612..5a7ca48 100644
--- a/polygerrit-ui/app/styles/shared-styles.ts
+++ b/polygerrit-ui/app/styles/shared-styles.ts
@@ -124,13 +124,8 @@
     /* iron-autogrow-textarea has a "-webkit-appearance: textarea" :host
         css rule, which prevents overriding the border color. Clear that. */
     -webkit-appearance: none;
-    --iron-autogrow-textarea: {
-      box-sizing: border-box;
-      padding: var(--spacing-s);
-    };
     --iron-autogrow-textarea_-_box-sizing: border-box;
     --iron-autogrow-textarea_-_padding: var(--spacing-s);
-    --iron-autogrow-textarea_-_white-space: pre-wrap;
   }
   a {
     color: var(--link-color);