DevTools: Make the 'Add Watch' button work with a keyboard

The watches panel asynchronously shows the pane when a new watch is
added, but the panel must be showing before creating the editor
otherwise focus will not be moved properly.

Bug: 809908
Change-Id: Ieaf3805f937557a07f8554ede60bd3df8820ec0f
Reviewed-on: https://chromium-review.googlesource.com/1077652
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562701}

third_party/blink/renderer/devtools/front_end/sources/WatchExpressionsSidebarPane.js

@@ -83,8 +83,8 @@ Sources.WatchExpressionsSidebarPane = class extends UI.ThrottledWidget {
     this._watchExpressionsSetting.set(toSave);
   }
 
-  _addButtonClicked() {
-    UI.viewManager.showView('sources.watch');
+  async _addButtonClicked() {
+    await UI.viewManager.showView('sources.watch');
     this._createWatchExpression(null).startEditing();
   }