Fix svg/custom/draw-image-crash.html flakiness

dumpAsText() must go before notifyDone()

TBR=pdr

Review URL: https://codereview.chromium.org/184643002

git-svn-id: svn://svn.chromium.org/blink/trunk@168136 bbb929c8-8fbe-4397-9dbb-9b2b20218538