Move browser tests that require installation into their own files.

Now, no test suite other than phantomjs_test.dart and
dartium_test.dart assume that phantomjs or dartium (respectively) are
installed. content_shell is still assumed to be installed, since it's
very useful for non-compilation-specific tests. More of those tests
have been moved to use content shell rather than chrome.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1088453004