Move inspected target events calculation into TracingTimelineModel

BUG=361045
R=caseq@chromium.org

Review URL: https://codereview.chromium.org/311113002

git-svn-id: svn://svn.chromium.org/blink/trunk@175482 bbb929c8-8fbe-4397-9dbb-9b2b20218538