This workaround uses UpdateResource instead of staging buffers for
texture data updates. It improves performance for highly dynamic
textures, especially in some benchmarks, eg the turbulenz particle
demo.

Re-land, with fix to the issue with glGenerateMipmaps.

BUG=angle:729
BUG=365078

Change-Id: I4c9398e0645176c296bf95e35eab97a44eae4319
Reviewed-on: https://chromium-review.googlesource.com/221493Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>