When mapping uniform buffers for very large buffers, we could end
up copying past buffer bounds. This would trigger crashes on some
configs. We can fix this by clamping the copy size correctly.

BUG=chromium:659892

Change-Id: I9d1af984da34867692d4c7b8908c016ebec7a63b
Reviewed-on: https://chromium-review.googlesource.com/404931Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>