Timeline: move TracingTimelineModel.reset() call from startRecording() to _startRecordingWithCategories()

This is a follow-up to https://codereview.chromium.org/326693002/ (r175810)

BUG=361045
TBR=yurys

Review URL: https://codereview.chromium.org/324913003

git-svn-id: svn://svn.chromium.org/blink/trunk@175893 bbb929c8-8fbe-4397-9dbb-9b2b20218538