Revert of android: Do not call ContentViewCore when destroying RenderWidgetHostViewAndroid (https://codereview.chromium.org/151563005/)

Reason for revert:
Breaking Android tests.  If you don't call SetContentViewCore(NULL) we leave a reference to the RWHVA inside WindowAndroid's observers.  Also need to take care of the destructor path in RenderWidgetHostViewAndroid (does that have to avoid a potentially bad ContentViewCore as well?).  ContentViewCore should unset itself from RenderWidgetHostViewAndroid when it's no longer valid to call into it.

Original issue's description:
> android: Do not call ContentViewCore when destroying RenderWidgetHostViewAndroid
> 
> Reverse a change from https://codereview.chromium.org/26753005/ that caused
> crashes.
> 
> BUG=324341
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=249654

TBR=sievers@chromium.org,feng@chromium.org,powei@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=324341

Review URL: https://codereview.chromium.org/154233004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249737 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249737