Add automated test for WebVR visibility without flag

Automates one of our manual test cases that checks that the WebVR API
is not visible on webpages when both:
1. The flag to enable WebVR is not explicitly set
2. The page does not have a WebVR origin trial token

Bug: 753540, 728779
Change-Id: I8aed5638a808d8a0ef9170795194cb85f7c2b754
Reviewed-on: https://chromium-review.googlesource.com/610643
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Biao She <bshe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493905}
2 files changed