Include test_shell.gypi in webkit.gyp instead of maintaining a distinct

test_shell.gyp.  This allows the removal of webkit.gyp:pull_in_test_shell,
eliminating the circular relationship between webkit.gyp and test_shell.gyp.

The expected semantics of building all of test_shell during a webkit.gyp:All
build is performed are retained.

This change depends on GYP r781 (in Chromium at r38526) for GYP suffix
operator path section treatment.  It also depends on Chromium r38519 and
r38658 for common.gypi treatment of chromium_code.

BUG=33186
TEST=Xcode no longer holds stale project files in memory
Review URL: http://codereview.chromium.org/602006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@38664 0039d316-1c4b-4281-b951-d872f2087c98