Fix crash due to ~AshAssistantController.

AshAssistantController needs to be cleaned up while there is still a
valid Shell instance.

Will do follow up CL to add unit tests.

Bug: 835410
Change-Id: I44edcb86e224cb6ade956db564228eb66d5bad83
Reviewed-on: https://chromium-review.googlesource.com/1022528Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#552510}

ash/shell.cc

@@ -815,6 +815,7 @@ Shell::~Shell() {
   // These need a valid Shell instance to clean up properly, so explicitly
   // delete them before invalidating the instance.
   // Alphabetical. TODO(oshima): sort.
+  ash_assistant_controller_.reset();
   magnification_controller_.reset();
   tooltip_controller_.reset();
   event_client_.reset();