Prevent spinning up a layer tree view after closing

This patch is a prerequisite for
https://codereview.chromium.org/134623005/ which makes output surface
creation asynchronous.  In that patch, to prevent races between
asynchronously closing the RenderWidgetHost and creating an output
surface (which can't succeed if the host goes away), it destroys the
layer tree view during RenderWidget::DoDeferredClose/closeWidgetSoon.

Unfortunately, this causes issues in WebViewImpl which wants to assert
that it has a layer tree view in all cases (except where null layer tree
views are allowed).  To prevent this, don't let accelerated compositing
spin up (again, or for the first time) after the layer tree view has
been closed.

BUG=none

Review URL: https://codereview.chromium.org/362403003

git-svn-id: svn://svn.chromium.org/blink/trunk@177424 bbb929c8-8fbe-4397-9dbb-9b2b20218538