Move AppWindowGeometryCacheTest to extensions_unittests

Move AppWindowGeometryCacheTest to extensions_unittests

It was running in Chrome's unit_tests for historical reasons.

* Remove usage of Chrome's TestExtensionPrefs by instantiating our own
  simplified ExtensionPrefs.
* Move ShellAppSorting to NullAppSorting so it can be used for the above
  as well as app_shell.

BUG=397164
TEST=extensions_unittests

Committed: https://crrev.com/b9129716d465383c2bf08b2c550f5af1205b2232
Cr-Commit-Position: refs/heads/master@{#293768}

[James Cook, 2014-09-05 23:49:16]

jamescook@chromium.org changed reviewers:
+ rockot@chromium.org

[James Cook, 2014-09-05 23:49:16]

rockot, PTAL

After moving ShellAppSorting to share code it ended up being pretty
straightforward.

[Ken Rockot(use gerrit already), 2014-09-06 02:43:13]

lgtm

[James Cook, 2014-09-07 00:05:08]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-09-07 00:05:57]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamescook@chromium.org/549663002/1

[commit-bot: I haz the power, 2014-09-07 02:06:17]

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

[commit-bot: I haz the power, 2014-09-07 02:06:18]

Exceeded time limit waiting for builds to trigger.

[James Cook, 2014-09-08 18:01:46]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-09-08 18:02:27]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamescook@chromium.org/549663002/40001

[commit-bot: I haz the power, 2014-09-08 19:30:25]

Committed patchset #3 (id:40001) as 95719c18f408331383eabb6bd135d2878dc87104

[commit-bot: I haz the power, 2014-09-10 03:47:37]

Patchset 3 (id:??) landed as
https://crrev.com/b9129716d465383c2bf08b2c550f5af1205b2232
Cr-Commit-Position: refs/heads/master@{#293768}