Make extension registry code use sequences instead of FILE thread.

FILE thread is deprecated, run extension registry code in
SequencedTaskRunner.

Also change
CHECK(BrowserThread::CurrentlyOn(FOO)) pattern to DCHECK.

BUG=689520

Change-Id: I6555e021ee80418e67d24bfca168c6be077b4bf1
Reviewed-on: https://chromium-review.googlesource.com/607470
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493450}
3 files changed