Store the mapping of client ID to service ID in an array for IDs less
than 16k.  This greatly speeds up ID lookups for the general case, from
a 8% hotspot to <1% on the MotionMark WebGL benchmark.

BUG=angleproject:2188

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: I81401c8e87bdb885a0b773050c930a94bcb3fa45
Reviewed-on: https://chromium-review.googlesource.com/653252


Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514505}