Create GLES2 context synchronously.

This introduces the infrastructure for "synchronous IPCs", and uses it for
context creation. Though not strictly necessary for the current hack, it will be
needed for the command buffer implementation, as well as for implementing an EGL
layer.

BUG=333157
R=darin@chromium.org

Review URL: https://codereview.chromium.org/147083005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248804 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248804