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There were three problems:
1. We didn't multiply the number of vertices by the number of instances.
2. We didn't account for vertices written by previous draw calls.
3. We didn't round the vertex count down to the nearest number of whole
   primitives.

Fixes the WebGL 2 conformance test added here:
https://github.com/KhronosGroup/WebGL/pull/2604

Bug: 820639
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: I9566ad14aa6671eca127a1b7f68b73e6408acd10
Reviewed-on: https://chromium-review.googlesource.com/957743
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543008}