Include content/content_shell_and_tests.gyp:* for all platforms.

The content_unittests target used to be pulled in by content.gyp:*
but after a refactoring it is necessary to explicitly include it via
content/content_shell_and_tests.gyp:*

BUG=None
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/48793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231394 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231394