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This applies only to code running under D3DVsync experiment.

My experiments show that increasing the GPU V-sync worker thread priority has 
a noticeable impact on latency on the GPU process side, even before the v-sync
signal is sent over IPC to the browser.

The tables below show the deltas (in us) between the adjusted DWM v-blank timestamp
and the now() time when on the thread wakeup on v-blank.
This was tested on a consumer grade DELL XPS laptop with 4 cores.

With the current DISPLAY priority the distribution looks like this:
< 0       9.8%
0-99      78.7%
100-199   4.8%
200-499   1.7%
500-999   2.2%
1000-1999 1.4%
2000-4999 1.0%
5000-9999 0.4%
10000+    <0.1%

With the higher REALTIME_AUDIO priority the distribution looks much improved
with a much steeper drop after 200 us:
< 0       8.6%
0-99      82.4%
100-199   8.5%
200-499   0.2%
500-999   0.2%
1000-1999 <0.1%

Bug: 751340
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I082d4561e6b27fc23e2242a9023c19d6787492fd
Reviewed-on: https://chromium-review.googlesource.com/596625Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Stanislav Chiknavaryan <stanisc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491528}