Fix snapshot canvas log test.

Review URL: https://codereview.chromium.org/320423004

BUG=381471

Review URL: https://codereview.chromium.org/321053005

git-svn-id: svn://svn.chromium.org/blink/trunk@175948 bbb929c8-8fbe-4397-9dbb-9b2b20218538