Restore samplers' state across virtualized GL contexts.

Tested with new WebGL conformance test being added in
https://github.com/KhronosGroup/WebGL/pull/2383 .

BUG=713127
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

Review-Url: https://codereview.chromium.org/2837213003
Cr-Original-Commit-Position: refs/heads/master@{#467520}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 69c721ec714b0650dec773173ef4be5a25347a49
12 files changed