When BufferVk::acquireAndUpdate calls into acquireBufferHelper
to allocate a new buffer helper we were passing in the update
size instead of the full buffer size.

Modified acquireAndUpdate's parameter to better reflect intent.

Bug: angleproject:5689
Change-Id: Ic4fbc015651491ec028d747da5d45670264b93fa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2746066Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>