[Extensions Toolbar Mac] Fix crash in browser action context menus

Right now, there's a chance that the NSMenu can outlive the underlying menu
model (which is owned by the view controller). Make sure we reset the context
menu before destruction and before creating a new menu to correct this.

BUG=462175

Review URL: https://codereview.chromium.org/967883006

Cr-Commit-Position: refs/heads/master@{#319678}

chrome/browser/ui/cocoa/extensions/browser_action_button.mm

@@ -411,6 +411,9 @@ void ToolbarActionViewDelegateBridge::DoShowContextMenu() {
   // We only need to do this because in Cocoa's memory management, removing the
   // button from the toolbar doesn't synchronously dealloc it.
   viewControllerDelegate_.reset();
+  // Also reset the context menu, since it has a dependency on the backing
+  // controller (which owns its model).
+  contextMenuController_.reset();
 }
 
 - (BOOL)isAnimating {
@@ -465,6 +468,9 @@ void ToolbarActionViewDelegateBridge::DoShowContextMenu() {
   if (testContextMenu_)
     return testContextMenu_;
 
+  // Make sure we delete any references to an old menu.
+  contextMenuController_.reset();
+
   ui::MenuModel* contextMenu = viewController_->GetContextMenu();
   if (!contextMenu)
     return nil;