TimelineFlameChart: reset main view when user starts recording new timeline

otherwise _selectedEntryIndex keeps its value and flame chart generates exception.

BUG=

Review URL: https://codereview.chromium.org/275883003

git-svn-id: svn://svn.chromium.org/blink/trunk@174018 bbb929c8-8fbe-4397-9dbb-9b2b20218538