Timeline: include records before first frame when left window selector is before first frame

We used to include all records before first frame as part of the first
frame, so selecting first frame in overview magically resulted in all
records from the start of timeline being included. Now we only start
frame explicitly since first BeginFrame signal, so if the page did not
redraw, the part of timeline before first frame would be impossible to
select by overview window. Let's include everything since the start of
timeline if the left window boundary is snapped to left. Also, fix similar
logic for snapping to right.

BUG=

Review URL: https://codereview.chromium.org/219963008

git-svn-id: svn://svn.chromium.org/blink/trunk@170946 bbb929c8-8fbe-4397-9dbb-9b2b20218538