ash: MagnificationControllerImpl -> MagnificationController

changes:
Mostly mechanical changes. Eliminate MagnificationControllerImpl and
make the methods the same order in .cc and .h.

Bug: none
Test: manual test and test coverages
Change-Id: I549aeb9722b9b86690de6db28c1dd0d98da68f6d
Reviewed-on: https://chromium-review.googlesource.com/972606
Commit-Queue: Qiang Xu <warx@google.com>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544767}

ash/shell.cc

@@ -1090,7 +1090,7 @@ void Shell::Init(ui::ContextFactory* context_factory,
   voice_interaction_controller_ =
       std::make_unique<VoiceInteractionController>();
 
-  magnification_controller_.reset(MagnificationController::CreateInstance());
+  magnification_controller_ = std::make_unique<MagnificationController>();
   mru_window_tracker_ = std::make_unique<MruWindowTracker>();
 
   autoclick_controller_.reset(AutoclickController::CreateInstance());