[TaskScheduler] Migrate ValidateCrxHelper off FILE thread.

FILE thread is deprecated, use SequencedTaskRunner instead.
--validate-crx is fairly isolated from regular browsing,
however since there is a very very small change of file
read colliding with other extension system file access,
use ExtensionFileTaskRunner to be safe.

Bug: 689520
Test: ./out/Release/chrome --validate-crx=crx_file_path With valid, invalid and non-existent file.

Change-Id: I06e7ad5a4bba5cab3c86d470e891c4b6fe81cee4
Reviewed-on: https://chromium-review.googlesource.com/610491
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494543}
1 file changed