This patch intends to solve several design issues by refactoring
the process of syncState in VertexArrayGL before implementing
ES3.1 feature Vertex Attrib Binding on the OpenGL back-end.
1. Use nullptr as the flag of using client memory pointer
2. Simplify comparisons in updateAttribPointer.
3. Put all code related to mFunctions->vertexAttrib*Pointer()
into an individual function
4. Remove redundant mStateManager->bindVertexArray() in all
update* functioins and only call it once in syncState().

BUG=angleproject:1593

Change-Id: I8f68534bb9291a601b9b77954d7281e5171c2b55
Reviewed-on: https://chromium-review.googlesource.com/465378
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>