Fix a missing dependency missing extensions.gyp.

This is due to the include of "extensions/grid/extensions_resources.h" in extension_api.cc. The missing this dependency was already introduced in gn build (https://codereview.chromium.org/596003002/) but not in the extensions.gyp.

The missing dependency will fail to build app_shell.

BUG=352293
Committed: https://crrev.com/46c567aecd2b80239fee6fc4006474db9c281229
Cr-Commit-Position: refs/heads/master@{#297860}

[Xi Han, 2014-10-02 14:07:33]

hanxi@chromium.org changed reviewers:
+ fsamuel@chromium.org, rockot@chromium.org

[Fady Samuel, 2014-10-02 14:20:38]

lgtm

[Ken Rockot(use gerrit already), 2014-10-02 17:08:47]

lgtm

[Xi Han, 2014-10-02 17:48:10]

The CQ bit was checked by hanxi@chromium.org

[commit-bot: I haz the power, 2014-10-02 17:48:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/619303002/1

[commit-bot: I haz the power, 2014-10-02 17:55:31]

Committed patchset #1 (id:1) as 2daed45338ecb50336332a28f60b81022f9dde9c

[commit-bot: I haz the power, 2014-10-02 17:56:18]

Patchset 1 (id:??) landed as
https://crrev.com/46c567aecd2b80239fee6fc4006474db9c281229
Cr-Commit-Position: refs/heads/master@{#297860}