We acquire a new BufferHelper from the pool when the app updates the
data of the entire buffer. In scenarios where the app updates say, 60%
of the buffer it would still be benificial to acquire a new buffer and
copy over the remaining 40% of data from the old buffer to the new one.
This reduces the transfer workload from 60% to 40% of buffer size.
Currently the threshold is set to 50% of buffer size.

Bug: angleproject:4380
Change-Id: I12576c585230e771d4c1a4352fab93dd3db2ecef
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2204655
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>