This makes the Context more available for logging errors.

Also includes more refactoring to VertexDataManager to ensure we can
access the gl::Context.

Bug: angleproject:2738
Change-Id: Iae3d22a1403078d236bfe63a3e2d203c13678dc4
Reviewed-on: https://chromium-review.googlesource.com/1151449Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>