Revert of bluetooth: Move requestDevice() to UI thread. (patchset #2 id:20001 of...

Revert of bluetooth: Move requestDevice() to UI thread. (patchset #2 id:20001 of https://codereview.chromium.org/1058373002/)

Reason for revert:
The layout test bluetooth/requestDevice.html became flaky after this patch landed.

Flakiness dashboard link:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=bluetooth%2FrequestDevice.html

Example failure link:
http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%2032/builds/35097

Failure output:
This is a testharness.js-based test.
PASS window.testRunner is required for the following tests.
PASS Reject with NotFoundError.
FAIL Reject with SecurityError. assert_unreached: Mock was set to reject. Reached unreachable code
PASS Mock will resolve.
Harness: the test ran to completion.

Original issue's description:
> bluetooth: Move requestDevice() to UI thread.
>
> BluetoothAdapter is not thread safe, this change moves the requestDevice() call to it onto the UI thread.
>
> BUG=420284
>
> Committed: https://crrev.com/8a72921779123fbd7a7f756f9683f4df8944ea9d
> Cr-Commit-Position: refs/heads/master@{#323919}

TBR=scheib@chromium.org,ortuno@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=420284

Review URL: https://codereview.chromium.org/1048313003

Cr-Commit-Position: refs/heads/master@{#323997}