Add autocomplete to iron-autogrow-textarea elements

Previously, iron-autogrow-textareas did not prompt autocomplete on
mobile devices. With this change, mobile browsers that support
autocomplete will suggest words to the user.

Bug: Issue 4683
Change-Id: I9286b998df95798f88c56f66fd63d731e5a88cc5

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

@@ -59,6 +59,7 @@ limitations under the License.
         <iron-autogrow-textarea
             id="messageInput"
             class="message"
+            autocomplete="on"
             placeholder="<Insert reasoning here>"
             bind-value="{{message}}"></iron-autogrow-textarea>
       </div>

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

@@ -71,6 +71,7 @@ limitations under the License.
         <iron-autogrow-textarea
             id="messageInput"
             class="message"
+            autocomplete="on"
             rows="4"
             max-rows="15"
             bind-value="{{message}}"></iron-autogrow-textarea>

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

@@ -55,8 +55,9 @@ limitations under the License.
         </label>
         <iron-autogrow-textarea
             id="messageInput"
-            max-rows="15"
             class="message"
+            autocomplete="on"
+            max-rows="15"
             bind-value="{{message}}"></iron-autogrow-textarea>
       </div>
     </gr-confirm-dialog>

polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html

@@ -199,6 +199,7 @@ limitations under the License.
         <iron-autogrow-textarea
             id="textarea"
             class="message"
+            autocomplete="on"
             placeholder="Say something..."
             disabled="{{disabled}}"
             rows="4"

polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.html

@@ -179,6 +179,7 @@ limitations under the License.
       <iron-autogrow-textarea
           id="editTextarea"
           class="editMessage"
+          autocomplete="on"
           disabled="{{disabled}}"
           rows="4"
           bind-value="{{_messageText}}"

polygerrit-ui/app/elements/settings/gr-ssh-editor/gr-ssh-editor.html

@@ -109,6 +109,7 @@ limitations under the License.
           <span class="value">
             <iron-autogrow-textarea
                 id="newKey"
+                autocomplete="on"
                 bind-value="{{_newKey}}"
                 placeholder="New SSH Key"></iron-autogrow-textarea>
           </span>

polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content.html

@@ -43,6 +43,7 @@ limitations under the License.
     </div>
     <div class="editor" hidden$="[[!editing]]">
       <iron-autogrow-textarea
+          autocomplete="on"
           bind-value="{{_newContent}}"
           disabled="[[disabled]]"></iron-autogrow-textarea>
       <div class="editButtons">