Unreviewed gardening: mark image only failures on scrollbar arrows.

The auto rebaseline in r165996 seems to have pixel diffs on our
builder. Marking as failing for tkent and ojan to look at later.

TBR=tkent@chromium.org,ojan@chromium.org
BUG=335472

Review URL: https://codereview.chromium.org/149093006

git-svn-id: svn://svn.chromium.org/blink/trunk@166023 bbb929c8-8fbe-4397-9dbb-9b2b20218538