content: Add layout_test_helper target.

This should build the layout test files that were copied from Blink over here.

BUG=331304
TEST=builds
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/124493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243043 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243043