This uses the most simple implementation: on BufferData calls without
explicit data arguments, it will initialize the buffer data store to
zero. This could be improved by deferring the init until needed, and
skipping it if the buffer store is cleared through other API calls,
but it is not a regression from current Chromium implementation.

BUG=angleproject:1635

Change-Id: I2fb1594851c5050dc2578736c3f74761555da267
Reviewed-on: https://chromium-review.googlesource.com/450921Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>