Create separate extensions_common and extensions_browser GYP targets.

BUG=162530
R=benwells@chromium.org

Review URL: https://codereview.chromium.org/23513035

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225260 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225260