Mark inspector/layers/layer-canvas-log.html as flaky.

BUG=381471
TBR=peter@chromium.org

Review URL: https://codereview.chromium.org/317283002

git-svn-id: svn://svn.chromium.org/blink/trunk@175605 bbb929c8-8fbe-4397-9dbb-9b2b20218538