Make abandon placeholder text optional

If a user doesn’t want to enter a reason for abandoning a change,
they have to clear the textarea. Move the default text to a
placeholder.

Change-Id: I6b6d53e0275163015420f5cc807fa3fb04b27a30

polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.html

diff --git a/polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.html b/polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.html
index bb897d1..7366d74 100644
--- a/polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.html
+++ b/polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.html
@@ -54,6 +54,7 @@
         <iron-autogrow-textarea
             id="messageInput"
             class="message"
+            placeholder="<Insert reasoning here>"
             bind-value="{{message}}"></iron-autogrow-textarea>
       </div>
     </gr-confirm-dialog>

polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.js

diff --git a/polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.js b/polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.js
index b733dbf..0ce1cbb 100644
--- a/polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.js
+++ b/polygerrit-ui/app/elements/change/gr-confirm-abandon-dialog/gr-confirm-abandon-dialog.js
@@ -30,10 +30,7 @@
      */
 
     properties: {
-      message: {
-        type: String,
-        value: '<INSERT REASONING HERE>',
-      },
+      message: String,
     },
 
     _handleConfirmTap: function(e) {