Extensions: Split the constants from the extensions_common target into its own extensions_common_co…

Extensions: Split the constants from the extensions_common target into its own extensions_common_constants target.

The extensions constants are used in too many places and very hard to
get rid of. Making it into its own target so it can be included in all
builds, even when extensions are not enabled.

BUG=424746
Committed: https://crrev.com/cdced8a6809240a42b61875af5ff2c37b754ba07
Cr-Commit-Position: refs/heads/master@{#300335}

[Lei Zhang, 2014-10-18 00:06:03]

thestig@chromium.org changed reviewers:
+ yoz@chromium.org

[Lei Zhang, 2014-10-18 00:09:09]

The new target will be used in chrome/chrome_common.gypi next week.

[Yoyo Zhou, 2014-10-20 21:09:13]

LGTM

[Lei Zhang, 2014-10-20 21:12:08]

The CQ bit was checked by thestig@chromium.org

[commit-bot: I haz the power, 2014-10-20 21:13:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/652623003/1

[commit-bot: I haz the power, 2014-10-20 21:45:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-20 21:46:07]

Patchset 1 (id:??) landed as
https://crrev.com/cdced8a6809240a42b61875af5ff2c37b754ba07
Cr-Commit-Position: refs/heads/master@{#300335}