Reland: Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

Reland: Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

app_shell needs the chrome.storage.local extension API so the core storage implementation is moving to //extensions/browser/api. However, the "managed storage" API implementation needs to remain in Chrome.

* Introduce ExtensionsApiClient in extensions/browser/api
* Introduce ChromeExtensionsApiClient in chrome/browser/extensions/api
* Delegate out the storage ManagedValueStoreCache creation to Chrome
* Move settings_namespace.h/cc to e/b/api/storage so we can use an enum in the client interface definition

Most APIs will probably need to make only one or two client interface calls, so we're starting with a single ExtensionsApiClient interface. It we find individual APIs tend to need a lot of methods we can split it into one client interface per API, but that seems like overkill right now.

Reland of https://codereview.chromium.org/180213008/ with fixes for Android GCMProfileServiceSingleProfileTest.Unregister

BUG=348058
TEST=unit_tests *Settings* and browser_tests ExtensionSettings*
TBR=rdevlin.cronin@chromium.org, rockot@chromium.org, yoz@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255384

[James Cook, 2014-03-05 23:53:11]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-03-06 00:26:10]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamescook@chromium.org/184853012/30026

[commit-bot: I haz the power, 2014-03-06 01:20:26]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-06 01:20:27]

Retried try job too often on win_rel for step(s) base_unittests, browser_tests,
interactive_ui_tests, net_unittests, unit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[James Cook, 2014-03-06 02:57:48]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-03-06 03:07:26]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamescook@chromium.org/184853012/30026

[commit-bot: I haz the power, 2014-03-06 06:35:40]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-06 06:35:41]

Retried try job too often on win_rel for step(s) unit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[James Cook, 2014-03-06 16:32:46]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-03-06 16:33:38]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jamescook@chromium.org/184853012/30026

[commit-bot: I haz the power, 2014-03-06 17:36:56]

Change committed as 255384