Rename RootWindowObserver -> WindowTreeHostObserver.

Move observer management and notification to WTH.

R=sky@chromium.org
http://crbug.com/348874

Review URL: https://codereview.chromium.org/186423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255108 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255108
Check some finalization properties.

  - Classes that have a user-declared destructor must inherit from a finalized GC base.
  - Destructors in GC derived class must not access GC managed fields.

BUG=334149
R=ager@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/187483004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255109 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255109