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
Committed: https://crrev.com/3a7f1e35efc1b70975193ec4b475e74b59bdf63c
Cr-Commit-Position: refs/heads/master@{#359021}

[Reilly Grant (use Gerrit), 2015-11-10 23:55:42]

reillyg@chromium.org changed reviewers:
+ rockot@chromium.org

[Reilly Grant (use Gerrit), 2015-11-10 23:55:42]

PTAL

[Ken Rockot(use gerrit already), 2015-11-11 00:44:15]

lgtm

[Reilly Grant (use Gerrit), 2015-11-11 00:44:57]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2015-11-11 00:45:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1419213006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1419213006/1

[commit-bot: I haz the power, 2015-11-11 02:28:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-11 02:28:47]

Patchset 1 (id:??) landed as
https://crrev.com/3a7f1e35efc1b70975193ec4b475e74b59bdf63c
Cr-Commit-Position: refs/heads/master@{#359021}