Create separate extensions_common and extensions_browser GYP targets.

BUG=162530
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Review URL: https://codereview.chromium.org/23513035

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225260 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed