WebVR: Fix Cardboard button processing

In native WebVR mode on a Cardboard style headset with capacitive touch
button, need to forward these touch events to the Javascript application
for processing, and emulate them for Daydream headsets.

BUG=389343,666467

Review-Url: https://codereview.chromium.org/2510083004
Cr-Commit-Position: refs/heads/master@{#433002}
1 file changed