Handle null Handler case in ContentViewRenderView

In certain edge cases the Handler for a View can be null (specifically when it is
detached from the window).  In this case we should rely on View#post() as that
handles the case internally.

BUG=304906

Review URL: https://codereview.chromium.org/27069004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228543 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228543