Move extensions manifest handler for OAuth2 to //extensions

Move extensions manifest handler for OAuth2 to //extensions

app_shell needs the client ID and oauth2 scopes for its identity API.

Just a refactor, no functional changes.

BUG=424653
TEST=extensions_unittests
TBR=xiyuan@chromium.org for header file rename touching chrome/browser/signin/easy_unlock_toggle_flow.cc

Committed: https://crrev.com/71187e343e2a8005dff92649b3013ba5019f974a
Cr-Commit-Position: refs/heads/master@{#300380}

[James Cook, 2014-10-20 21:47:17]

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

[James Cook, 2014-10-20 21:47:18]

rockot, PTAL

[Ken Rockot(use gerrit already), 2014-10-20 22:55:31]

lgtm

[James Cook, 2014-10-20 23:03:03]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-10-20 23:04:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/671553002/1

[commit-bot: I haz the power, 2014-10-20 23:47:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-20 23:48:13]

Patchset 1 (id:??) landed as
https://crrev.com/71187e343e2a8005dff92649b3013ba5019f974a
Cr-Commit-Position: refs/heads/master@{#300380}