This adds helper functions to handle common use case for onBufferRead
and onBufferWrite to reduce the API verbosity a little bit. Also fix the
transform feedback bug that we are passing in wrong access/stage flags
when it is emulated by vertex shader.

Bug: b/155122200
Change-Id: Id2549ca00cad184a90c6230dc3665aaff44dda08
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2174265
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>