Pepper: Crash fix for FlushCommandBuffer().

PPB_VideoDecoder_Shared uses ResourceTracker to hold a "plugin reference" on a
Graphics3D resource to keep it alive, since it uses Graphics3D's gles2_impl().

However, on instance destruction, resources are destroyed regardless of how
"plugin references" are held.

This change checks that the Graphics3D resource is still live before using its
gles2_impl_.

A better fix would clean this up to use scoped_refptr<> instead of using
"plugin references".

BUG=263235

Review URL: https://codereview.chromium.org/99763006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240208 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240208