Perf Tests: Compute GPU time periodically If you specify `--fixed-test-time 30` while running app perftest and watch for GPU memory allocation, you see GPU memory keeps growing. This is due to we generate timestamp query for every frame, but don't calculate the GPU time at the end of run. We accumulate all these queries over the entire test run. Even though this is technically not a leak, it does grow monotonically while test is running. This CL calls computeGPUTime every 16 frames (steps) and changed the mTimestampQueries to a queue structure and clean up the completed query as GPU time is computed. Bug: angleproject:7243 Change-Id: I283ee24c4835fe0fb4d15f8d1975c85b64e16443 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3609011 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com>