Check if RTI::SharedMainThreadContextProvider is there before wrapping

It's not valid to wrap a NULL cc::ContextProvider in a
WebGraphicsContext3DProvider, so NULL check it before wrapping. The
SharedMainThreadContextProvider() could be NULL if we can't create
and bind a valid context, which can happen if the GPU is unhappy or
the process crashes any early point in initialization.

BUG=301555

Review URL: https://codereview.chromium.org/26028004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227166 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227166