Update extensions_resources to use grit_target.gypi.

This is just a small cleanup while I'm here and for being consistent
with other files of the same nature.

BUG=None
TEST=extensions_unittests
R=yoz@chromium.org

Review URL: https://codereview.chromium.org/270243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268962 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed