Make extensions_utility target properly depend on extensions_strings

Make extensions_utility target depend on extensions_strings

This has been causing occasional build breakage on the bots,
depending on whether we got unlucky in build ordering. Because
extensions_common and extensions_browser both cause
extensions_strings to run, I didn't notice this error before.

BUG=451442
Committed: https://crrev.com/33f39fe7413561f88f312e4db3e1c2e5b9fa1b5a
Cr-Commit-Position: refs/heads/master@{#312957}

[asargent_no_longer_on_chrome, 2015-01-23 21:22:56]

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

[Ken Rockot(use gerrit already), 2015-01-23 21:23:54]

too complicated not lgtm

[Ken Rockot(use gerrit already), 2015-01-23 21:24:44]

ok fine. lgtm

[asargent_no_longer_on_chrome, 2015-01-23 21:34:17]

The CQ bit was checked by asargent@chromium.org

[commit-bot: I haz the power, 2015-01-23 21:35:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/869743004/1

[commit-bot: I haz the power, 2015-01-23 22:56:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-23 22:58:18]

Patchset 1 (id:??) landed as
https://crrev.com/33f39fe7413561f88f312e4db3e1c2e5b9fa1b5a
Cr-Commit-Position: refs/heads/master@{#312957}