TimelinePanel: move windowStartTime/windowEndTime mechanic from TimelineView to TimelinePanel.

BUG=326121
R=caseq@chromium.org

Review URL: https://codereview.chromium.org/150373002

git-svn-id: svn://svn.chromium.org/blink/trunk@166310 bbb929c8-8fbe-4397-9dbb-9b2b20218538