Add KeyedServiceShutdownNotifier for classes that depend on keyed services but can't be keyed servi…

Add KeyedServiceShutdownNotifier for classes that depend on keyed services but can't be keyed services themselves.

BUG=395391
Committed: https://crrev.com/76e4425f572bdbe42a8564c0ceb110c3b24db9f9
Cr-Commit-Position: refs/heads/master@{#315234}

[Bernhard Bauer, 2015-02-06 16:10:12]

bauerb@chromium.org changed reviewers:
+ erg@chromium.org

[Bernhard Bauer, 2015-02-06 16:20:35]

Hi Elliot, 

PTAL at this CL and let me know what you think.

For a concrete use case for this class, see
https://codereview.chromium.org/901573003/ and the associated
https://crbug.com/395391: A BrowserMessageFilter usually lives as long as the
RenderProcessHost it's attached to (and there can be more than one per browser
context), but because it's refcounted and receives messages on the IO thread but
handles them on the UI thread, the posted task might run after the profile has
been shutdown, so it needs to be notified about that.

Other solutions for this are either somewhat deprecated (using notifications) or
feel really hacky (checking ProfileManager::IsValidProfile), so I figured the
obvious choice would be the KeyedService mechanism. 

One thing I'm not completely sure about is whether declaring individual
dependencies might be overkill here, as all keyed services are shut down during
a single message loop iteration anyway, so if the message handling task runs,
either the profile and all keyed services are going to be alive, or neither. But
then again, I kind of like not directly depending on the profile...

[Elliot Glaysher, 2015-02-06 20:59:27]

i suppose this lgtm

it feels more than a little hacky to put things that aren't actually services in
the service graph, but I understand that you need to have adapters/notifiers
which bridge non-keyedcontextfactory code.

[Bernhard Bauer, 2015-02-08 13:53:55]

The CQ bit was checked by bauerb@chromium.org

[commit-bot: I haz the power, 2015-02-08 13:54:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/905703002/1

[commit-bot: I haz the power, 2015-02-08 14:02:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-08 14:03:28]

Patchset 1 (id:??) landed as
https://crrev.com/76e4425f572bdbe42a8564c0ceb110c3b24db9f9
Cr-Commit-Position: refs/heads/master@{#315234}