Require secure contexts and user gestures to use WebUSB.
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#359021}

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