Mark recently added inspector timeline test as flaky.

TBR=yurys@chromium.org
BUG=412782

Review URL: https://codereview.chromium.org/599813002

git-svn-id: svn://svn.chromium.org/blink/trunk@182562 bbb929c8-8fbe-4397-9dbb-9b2b20218538