Replace css mixins in gr-editable-label by ::part()

Change-Id: Id48ead3d7f2cb47cf1eb5a6b58441f20ff859a97
diff --git a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view_html.ts b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view_html.ts
index 4761037..b577db3 100644
--- a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view_html.ts
+++ b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view_html.ts
@@ -40,14 +40,14 @@
       font-size: var(--font-size-h3);
       font-weight: var(--font-weight-h3);
       line-height: var(--line-height-h3);
-      --label-style: {
-        text-overflow: initial;
-        white-space: initial;
-        word-break: break-all;
-      }
-      --input-style: {
-        margin-top: var(--spacing-l);
-      }
+    }
+    header gr-editable-label::part(label) {
+      text-overflow: initial;
+      white-space: initial;
+      word-break: break-all;
+    }
+    header gr-editable-label::part(input-container) {
+      margin-top: var(--spacing-l);
     }
     .textareaWrapper {
       border: 1px solid var(--border-color);
diff --git a/polygerrit-ui/app/elements/shared/gr-editable-label/gr-editable-label_html.ts b/polygerrit-ui/app/elements/shared/gr-editable-label/gr-editable-label_html.ts
index a227482..c303bed 100644
--- a/polygerrit-ui/app/elements/shared/gr-editable-label/gr-editable-label_html.ts
+++ b/polygerrit-ui/app/elements/shared/gr-editable-label/gr-editable-label_html.ts
@@ -35,7 +35,6 @@
       overflow: hidden;
       text-overflow: ellipsis;
       white-space: nowrap;
-      @apply --label-style;
     }
     label.editable {
       color: var(--link-color);
@@ -47,7 +46,6 @@
     .inputContainer {
       background-color: var(--dialog-background-color);
       padding: var(--spacing-m);
-      @apply --input-style;
     }
     .buttons {
       display: flex;
@@ -83,6 +81,7 @@
       title$="[[_computeLabel(value, placeholder)]]"
       aria-label$="[[_computeLabel(value, placeholder)]]"
       on-click="_showDropdown"
+      part="label"
       >[[_computeLabel(value, placeholder)]]</label
     >
   </template>
@@ -104,7 +103,7 @@
     on-iron-overlay-canceled="_cancel"
   >
     <div class="dropdown-content" slot="dropdown-content">
-      <div class="inputContainer">
+      <div class="inputContainer" part="input-container">
         <template is="dom-if" if="[[!autocomplete]]">
           <paper-input
             id="input"