time the GPU force clearing async launch overhead

Unfortunately, the asynchronous launch of GPU force buffer clearing can
take up to 2% of the total run-time with short iteration times and
many/fast cores/GPU. Timing it will at least remove it form the "Rest".

Change-Id: I397c563ead24d87181de1b03879f164d1a97c2ca
Merge "time the GPU force clearing async launch overhead" into release-4-6