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As a workaround for M62, we were setting default context title
|display_source| for some specific notifications.
For the detailed background, see the original CL
https://crrev.com/c/662501 and the bug https://crbug.com/761900 .

As all the caller side fixes have been landed, and the workaround
is merged to M62 release branch, we can now remove this.

Caller side fix CLs:
* ARC notifications
  https://crrev.com/c/647391
  https://crrev.com/c/654119
* Sign-in error notifications
  https://crrev.com/c/648891
  https://crrev.com/c/657138
* HaTS notification https://crrev.com/c/646413
* CUPS notification https://crrev.com/c/648539

TEST=manual
BUG=none

Change-Id: Ie358b8a2070ff3870629d0b6d46e2a2b2fc9f5ba
Reviewed-on: https://chromium-review.googlesource.com/662485Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501543}