Revert "Switch browser/extensions/api/system_storage/system_storage_api.[h|cc] to the TaskScheduler API"

This reverts commit 73d162eb24c9c7a6b7cf4b65bc0562ae0a4236d9.

Reason for revert: Suspected cause of data race in ExtensionServiceTest.StorageQuota: https://build.chromium.org/p/chromium.memory/builders/Linux%20TSan%20Tests/builds/9501

Original change's description:
> Switch browser/extensions/api/system_storage/system_storage_api.[h|cc] to the TaskScheduler API
> 
> BUG=689520
> 
> Change-Id: Ia07f6c50a4dd0df96a63194e78a4cda85df181c6
> Reviewed-on: https://chromium-review.googlesource.com/586467
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#490406}

TBR=gab@chromium.org,benhenry@chromium.org,ningxin.hu@intel.com

Change-Id: Iaafab4821c676cb077fdfca970f534a2bfcb0114
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 689520
Reviewed-on: https://chromium-review.googlesource.com/591933
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490534}
5 files changed