Create gr-apply-fix-dialog and show fix preview.

in this change:
- add "Show Fix" option to gr-comment when fixes exist
- add GET /preview and POST /apply endpoints to gr-rest-api-interface
- create gr-apply-fix-dialog that fetch preview and supports showing
multiple diffs for a single fix, apply fix button that navigate to
edit-change view, and cancel that close dialog

Screenshots: https://imgur.com/a/6xQIoz8

next:
- support switching between multiple fixes

Change-Id: I9ba5121dd3b5b51791540ac9758f98c8669b44b2

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

@@ -358,6 +358,15 @@ limitations under the License.
                 Please Fix
               </gr-button>
             </template>
+            <gr-button
+                link
+                secondary
+                class="action show-fix"
+                hidden$="[[_hasNoFix(comment)]]"
+                on-click="_handleShowFix"
+                disabled="[[robotButtonDisabled]]">
+              Show Fix
+            </gr-button>
             <gr-endpoint-decorator name="robot-comment-controls">
               <gr-endpoint-param name="comment" value="[[comment]]">
               </gr-endpoint-param>

polygerrit-ui/app/elements/shared/gr-comment/gr-comment.js

@@ -48,6 +48,12 @@
      * @event create-fix-comment
      */
 
+    /**
+     * Fired when the show fix preview action is triggered.
+     *
+     * @event open-fix-preview
+     */
+
     /**
      * Fired when this comment is discarded.
      *
@@ -505,6 +511,18 @@
       }));
     }
 
+    _handleShowFix() {
+      this.dispatchEvent(new CustomEvent('open-fix-preview', {
+        bubbles: true,
+        composed: true,
+        detail: this._getEventPayload(),
+      }));
+    }
+
+    _hasNoFix(comment) {
+      return !comment || !comment.fix_suggestions;
+    }
+
     _handleDiscard(e) {
       e.preventDefault();
       this.$.reporting.recordDraftInteraction();

polygerrit-ui/app/elements/shared/gr-comment/gr-comment_test.html

@@ -1007,5 +1007,17 @@ limitations under the License.
       flushAsynchronousOperations();
       assert.isNotNull(element.$$('.robotActions gr-button'));
     });
+
+    test('_handleShowFix fires open-fix-preview event', done => {
+      element.addEventListener('open-fix-preview', e => {
+        assert.deepEqual(e.detail, element._getEventPayload());
+        done();
+      });
+      element.comment = {fix_suggestions: [{}]};
+      element.isRobotComment = true;
+      flushAsynchronousOperations();
+
+      MockInteractions.tap(element.$$('.show-fix'));
+    });
   });
 </script>

polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js

@@ -1860,6 +1860,25 @@
       });
     }
 
+    getRobotCommentFixPreview(changeNum, patchNum, fixId) {
+      return this._getChangeURLAndFetch({
+        changeNum,
+        patchNum,
+        endpoint: `/fixes/${encodeURIComponent(fixId)}/preview`,
+        reportEndpointAsId: true,
+      });
+    }
+
+    applyFixSuggestion(changeNum, patchNum, fixId) {
+      return this._getChangeURLAndSend({
+        method: 'POST',
+        changeNum,
+        patchNum,
+        endpoint: `/fixes/${encodeURIComponent(fixId)}/apply`,
+        reportEndpointAsId: true,
+      });
+    }
+
     // Deprecated, prefer to use putChangeCommitMessage instead.
     saveChangeCommitMessageEdit(changeNum, message) {
       return this._getChangeURLAndSend({