Mark virtual/implsidepainting/inspector/timeline/timeline-paint.html as flaky

this started when we added virtual/implsidepainting/inspector/timeline in r158321.

BUG=298188
TBR=loislo,pfeldman

Review URL: https://codereview.chromium.org/24611002

git-svn-id: svn://svn.chromium.org/blink/trunk@158329 bbb929c8-8fbe-4397-9dbb-9b2b20218538