Implement robust buffer initialization. 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/450921 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>