Add an app_shell_unittests target

Add an app_shell_unittests target

For now this uses extensions_unittests_main.cc as there are no differences
in startup between extensions_unittests and app_shell_unittests.

This allows ShellNaClBrowserDelegate to have a test.

BUG=402612
TEST=app_shell_unittests

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=290719

[James Cook, 2014-08-19 18:19:25]

derat, PTAL

[Daniel Erat, 2014-08-19 18:27:10]

lgtm

[James Cook, 2014-08-19 18:28:27]

yoz, PTAL.  I'm going to use this to test
http://codereview.chromium.org/477113006 (and add more tests there, especially
for NaCl stuff)

[Yoyo Zhou, 2014-08-19 18:49:31]

LGTM

[James Cook, 2014-08-19 21:14:29]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-08-19 21:14:58]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamescook@chromium.org/471453003/20001

[James Cook, 2014-08-19 21:29:44]

The CQ bit was unchecked by jamescook@chromium.org

[James Cook, 2014-08-19 21:37:23]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-08-19 21:37:57]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamescook@chromium.org/471453003/40001

[commit-bot: I haz the power, 2014-08-20 00:51:11]

Committed patchset #3 (40001) as 290719