Make the extensions button the default extensions bubble anchor in the new extensions container.

This will also cause the extensions button to highlight while a bubble is anchored.

Bug: 987312
Change-Id: Id2b3e831dcceeb5ac47b929078f51ac3ebaecf12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904492Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713645}

chrome/browser/ui/views/toolbar/toolbar_view.cc

@@ -756,7 +756,7 @@ ToolbarActionView* ToolbarView::GetToolbarActionViewForId(
 
 views::View* ToolbarView::GetDefaultExtensionDialogAnchorView() {
   if (extensions_container_)
-    return extensions_container_;
+    return extensions_container_->extensions_button();
   return GetAppMenuButton();
 }