Timeline: for idle frames, pick screenshots matching frame start time, not end time
Timeline: for idle frames, pick screenshots matching frame start time, not end time

Idle frame ends are forged, so there would be no matching screenshot. Since we
also produce no output for idle frames, we should show one for previous frame,
so use idle frame start time which matches the end time of last frame.

BUG=

Review URL: https://codereview.chromium.org/1290543002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200369 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk