Require secure contexts and user gestures to use WebUSB.

Adds checks in Blink to only allow secure contexts to call getDevices
and requestDevice. As requestDevice also displays a permission prompt it
also requires a user gesture.

BUG=492204

Review URL: https://codereview.chromium.org/1419213006

Cr-Commit-Position: refs/heads/master@{#359021}
1 file changed