When we move a BufferHelper to the mInFlightBuffers list
don't try to unmap it there, instead do that when it gets added
to mBufferFreeList

This removes 0.75% CPU overhead from a Manhattan30 offscreen run

Bug: angleproject:5689
Change-Id: I01a52cdf4803fb8364140b01bedbd3d0e6cc61fd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2723492
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>