DynamicBuffer are not cache coherent, thus requires flush. Also adds a
few assertion to ensure implementation matches expectation.

Bug: b/155432713
Change-Id: Iaf28786168a3bb5d746b43e030f882c4b6d005ad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2174269Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>