Fix nullptr dereference in MojoBindingsController

Fix nullptr dereference in MojoBindingsController

Based on the DCHECK in
RenderFrameImpl::runScriptsAtDocumentElementAvailable,
it's possible for |frame_| to be null when calling
MojoBindingsController::RunScriptsAtDocumentReady. The latter
assumes this isn't possible and uses the result of GetWebFrame()
without checking it.

This CL fixes that to avoid a potential nullptr dereference.

BUG=595875
R=jam@chromium.org

[Ken Rockot(use gerrit already), 2016-06-27 15:46:37]

Description was changed from

==========
Fix nullptr dereference in MojoBindingsController

Based on the DCHECK in
RenderFrameImpl::runSCriptsAtDocumentElementAvailable,
it's possible for |frame_| to be null when calling
MojoBindingsController::RunScriptsAtDocumentReady. The latter
assumes this isn't possible and uses the result of GetWebFrame()
without checking it.

This CL fixes that to avoid a potential nullptr dereference.

BUG=595875
R=jam@chromium.org
==========

to

==========
Fix nullptr dereference in MojoBindingsController

Based on the DCHECK in
RenderFrameImpl::runScriptsAtDocumentElementAvailable,
it's possible for |frame_| to be null when calling
MojoBindingsController::RunScriptsAtDocumentReady. The latter
assumes this isn't possible and uses the result of GetWebFrame()
without checking it.

This CL fixes that to avoid a potential nullptr dereference.

BUG=595875
R=jam@chromium.org
==========

[Ken Rockot(use gerrit already), 2016-06-27 20:58:09]

Will hold off on this for now. dcheng@ thinks it may be a badly behaved observer
detaching the frame and will confirm soonish.