Revert "Migrate file_system file manager APIs to UIThreadExtensionFunction."

This reverts commit 76efdcf8bb1fe30d3ba0f748128072edb522160f.

Reason for revert: Suspected of breaking
FileSystemProviderApiTest.RemoveWatcher
FileSystemProviderApiTest.AddWatcher
in browser_tests on linux-chromeos-rel

Example failure
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-rel/20439

Original change's description:
> Migrate file_system file manager APIs to UIThreadExtensionFunction.
> 
> BUG=934541
> 
> Change-Id: Ibd06bda730bd10c270d187d83f68315159fa6a43
> Reviewed-on: https://chromium-review.googlesource.com/c/1482172
> Reviewed-by: Sam McNally <sammc@chromium.org>
> Commit-Queue: Anand Mistry <amistry@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#634562}

TBR=sammc@chromium.org,amistry@chromium.org

Change-Id: I56ca79cb7a1d358539d31d3c1156859232e08ce3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 934541
Reviewed-on: https://chromium-review.googlesource.com/c/1482970
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634593}
2 files changed