arc: Fix thread unsafe behavior of ArcDownloadsWatcherService

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

[hashimoto, 2016-09-07 03:34:11]

Description was changed from

==========
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)
==========

to

==========
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/+/8c306683e2476a091d8fd0b94c87...
==========

[hashimoto, 2016-09-07 03:34:14]

Committed patchset #1 (id:1) manually as
8c306683e2476a091d8fd0b94c87a9ecefff87bd.

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

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/2315013003/ by hashimoto@chromium.org.

The reason for reverting is: Syntax error.