Revert of arc: Fix thread unsafe behavior of ArcDownloadsWatcherService

Revert of arc: Fix thread unsafe behavior of ArcDownloadsWatcherService (patchset #1 id:1 of https://codereview.chromium.org/2313213002/ )

Reason for revert:
Syntax error

Original issue's description:
> arc: Fix thread unsafe behavior of ArcDownloadsWatcherService
> 
> ArcBridgeService is not thread-safe.
> WeakPtr should be defererenced only on the thread to which the
> factory belongs.
> 
> BUG=643059
> 
> Review-Url: https://codereview.chromium.org/2304523002
> Cr-Commit-Position: refs/heads/master@{#416194}
> (cherry picked from commit 3c74f1cb2742e9b050fbc830f1ac0e06163c2370)
> 
> Committed: https://chromium.googlesource.com/chromium/src/+/8c306683e2476a091d8fd0b94c87a9ecefff87bd

TBR=
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=643059

[hashimoto, 2016-09-07 03:38:03]

The CQ bit was checked by hashimoto@chromium.org

[hashimoto, 2016-09-07 03:38:03]

Created Revert of arc: Fix thread unsafe behavior of ArcDownloadsWatcherService

[commit-bot: I haz the power, 2016-09-07 03:38:21]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-07 03:41:29]

Committed patchset #1 (id:1)