Support vertex data stored in client memory passed to glVertexAttribPointer.
Only GL_FLOAT data is supported at this time.  Includes a simple test.

BUG=angleproject:1683

Change-Id: I3bc0cdefe02b02c046b0e85822019a0f1762235e
Reviewed-on: https://chromium-review.googlesource.com/425137
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>