Allow app_shell embedders to override extensions client interfaces

Allow app_shell embedders to override extensions client interfaces

This includes ExtensionsClient and ExtensionsBrowserClient.

BUG=none
TEST=app_shell_unittests

Committed: https://crrev.com/4d8d72ef85260841830d4215f0fcfaf9d49fd5b6
Cr-Commit-Position: refs/heads/master@{#304946}

[James Cook, 2014-11-19 17:14:03]

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

[James Cook, 2014-11-19 17:14:03]

Dan, PTAL

[James Cook, 2014-11-19 17:27:32]

Actually, wait on this. It might need to be more elaborate.

[James Cook, 2014-11-19 21:52:23]

Dan, PTAL

[Daniel Erat, 2014-11-20 00:06:14]

lgtm

[James Cook, 2014-11-20 00:08:04]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-11-20 00:08:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/717233003/40001

[commit-bot: I haz the power, 2014-11-20 00:49:57]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2014-11-20 00:51:17]

Patchset 3 (id:??) landed as
https://crrev.com/4d8d72ef85260841830d4215f0fcfaf9d49fd5b6
Cr-Commit-Position: refs/heads/master@{#304946}