Gardening: make timeline-decode-resize.html on Mac timeout not slow.

BUG=372213
TBR=ojan
NOTRY=true

Review URL: https://codereview.chromium.org/282583002

git-svn-id: svn://svn.chromium.org/blink/trunk@173882 bbb929c8-8fbe-4397-9dbb-9b2b20218538