Convert a chrome_tests.gypi 'sources/' block to 'sources!'

The more expensive regex matching is not needed for explicit files.

BUG=NONE
TEST=NONE
R=ben@chromium.org

Review URL: https://codereview.chromium.org/27055008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229286 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229286