Shrink the reference image dimension in OffscreenCanvas tests

The android device has a screen resolution width of 1080 and
devicePixelRatio of 2.625. As a result, the width of 600 in the
existing gpu pixel tests is too big as width * devicePixelRatio
> screen resolution. Therefore, I shrink the test rect to 400 in
width.

BUG=697582
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/2774423006
Cr-Commit-Position: refs/heads/master@{#460237}
5 files changed