Move WebUSB from OriginTrialEnabled to RuntimeEnabled.

This reverses commit 5c00cb62536b679d52c3741598d3f08f79e7e32f.

BUG=605326, 492204

TEST=To verify:

On Windows, Chrome OS, Mac OSX:

Check the following with chrome launched without command line flags:

For each site:
 https://permission.site
 http://permission.site
Load and press the USB button.
 Expect in Chrome Developer Tools: Console output:
  "Uncaught TypeError: Cannot read property 'requestDevice' of undefined"

Check the following with chrome launched with command line flag:
 --enable-blink-features=ExperimentalFramework

For https://permission.site
Load and press the USB button.
 Expect a chooser dialog to open.

For http://permission.site
Load and press the USB button.
 Expect in Chrome Developer Tools: Console output:
  "error usb DOMException: Only secure origins are allowed (see: https://goo.gl/Y0ZkNV)."

Review-Url: https://codereview.chromium.org/1917103004
Cr-Commit-Position: refs/heads/master@{#391090}
(cherry picked from commit 403af928b91df81acac1fc3b27fc093f2eb6da24)

Review URL: https://codereview.chromium.org/1959143002 .

Cr-Commit-Position: refs/branch-heads/2704@{#449}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
18 files changed