Removed test case having scrollbar issues on Mac.

For some reason the generated scrollbar pseudo element count was different
on Mac, although the test passed on the Mac bot when landing. I've removed
the test for :single-button/:double-button as the test coverage by the
other pseudo classes should be enough.

R=leviw@chromium.org
BUG=427953

Review URL: https://codereview.chromium.org/687943002

git-svn-id: svn://svn.chromium.org/blink/trunk@185036 bbb929c8-8fbe-4397-9dbb-9b2b20218538