Use base::StringPairs where appropriate from src/extensions

Use base::StringPairs where appropriate from src/extensions

Becuase base/strings/string_split.h defines:
typedef std::vector<std::pair<std::string, std::string> > StringPairs;

BUG=412250
Committed: https://crrev.com/245433c30a0d2d0641c0ba699cbf6228c4a43e01
Cr-Commit-Position: refs/heads/master@{#296369}

[Anand Ratn (left samsung), 2014-09-23 13:38:29]

anand.ratn@samsung.com changed reviewers:
+ rockot@chromium.org

[Anand Ratn (left samsung), 2014-09-23 13:38:29]

rockot@chromium.org: Please review changes in src/extensions module

[Ken Rockot(use gerrit already), 2014-09-23 16:15:55]

lgtm

[Anand Ratn (left samsung), 2014-09-24 03:10:29]

The CQ bit was checked by anand.ratn@samsung.com

[commit-bot: I haz the power, 2014-09-24 03:11:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/594133003/1

[Anand Ratn (left samsung), 2014-09-24 05:00:37]

The CQ bit was unchecked by anand.ratn@samsung.com

[Anand Ratn (left samsung), 2014-09-24 08:51:39]

The CQ bit was checked by anand.ratn@samsung.com

[commit-bot: I haz the power, 2014-09-24 08:52:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/594133003/1

[commit-bot: I haz the power, 2014-09-24 09:15:10]

Committed patchset #1 (id:1) as b7b45f17fec3318ff82c900a36f7800e04c2a215

[commit-bot: I haz the power, 2014-09-24 09:15:45]

Patchset 1 (id:??) landed as
https://crrev.com/245433c30a0d2d0641c0ba699cbf6228c4a43e01
Cr-Commit-Position: refs/heads/master@{#296369}