Update the BufferStorage classes to use size_t for all sizes and offsets.
BufferStorage9 now uses a std::vector to manage its memory.

BUG=angle:631

Change-Id: Iea4e7b33ede59a5b9c6a1245690c4b7865096fc3
Reviewed-on: https://chromium-review.googlesource.com/197819Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>