Add default body to OnKeyboardClosed()

Almost all implementations of this method are empty so there isn't
really a good reason why it should be required.

Bug: 
Change-Id: I1e680280db194a8af6d00f689e70ad64c9a9062f
Reviewed-on: https://chromium-review.googlesource.com/792755Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Blake O'Hare <blakeo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519588}

ui/keyboard/keyboard_controller_observer.h

@@ -20,8 +20,9 @@ class KEYBOARD_EXPORT KeyboardControllerObserver {
  public:
   virtual ~KeyboardControllerObserver() {}
 
-  // DEPRECATED
   // Called when the keyboard bounds or visibility are about to change.
+  // DEPRECATED - please use one of the other more specific methods below. Once
+  // all callers of this method have been migrated, it will be removed.
   virtual void OnKeyboardBoundsChanging(const gfx::Rect& new_bounds) {}
 
   // Called when the keyboard is shown or closed.
@@ -42,7 +43,7 @@ class KEYBOARD_EXPORT KeyboardControllerObserver {
       const gfx::Rect& new_bounds){};
 
   // Called when the keyboard was closed.
-  virtual void OnKeyboardClosed() = 0;
+  virtual void OnKeyboardClosed(){};
 
   // Called when the keyboard has been hidden and the hiding animation finished
   // successfully. This is same as |state| == HIDDEN on OnStateChanged.