Add a new shared page state in pixel_tests.py

In a previous CL:https://codereview.chromium.org/2328463004/, there was
a pixel test that is added which test WebGL's commit() function. What
was missing in that CL is that the test requires an additional browser
arg: --enable-experimental-canvas-features. Without this flag, it is
causing timeout exception.

This CL adds a new shared page state with that flag, which should make
the bots correctly run this test and generate reference images.

BUG=563852
CQ_INCLUDE_TRYBOTS=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;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2355913002
Cr-Commit-Position: refs/heads/master@{#420033}
3 files changed