Testcase for crbug.com/350907

Also replace the FIXME with an attempt at explaining why/when m_image
can be 0.

BUG=350907

Review URL: https://codereview.chromium.org/198943004

git-svn-id: svn://svn.chromium.org/blink/trunk@169210 bbb929c8-8fbe-4397-9dbb-9b2b20218538