This is more in line with the rest of ANGLE which assumes a binding of 0
size means the size of gl::Buffer.  This allows
GetBoundBufferAvailableSize to be used in texture buffer related code.

Bug: angleproject:3573
Change-Id: I305835f305d4bf4422759bb96e030cbbc136871a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2542863
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>