Mark PinchViewportTest.TestScrollFocusedNodeIntoRect as flaky.

This was failing intermittently on the Blink Mac builders.

TBR=bokan@chromium.org
BUG=401151

Review URL: https://codereview.chromium.org/443173002

git-svn-id: svn://svn.chromium.org/blink/trunk@179649 bbb929c8-8fbe-4397-9dbb-9b2b20218538