Encapsulate implementation-specific timeline record handling in TimelineUIUtils

TimelineUIUtils now has two descendants:
- TracingTimelineUIUtils for tracing based timeline records
- TimelineUIUtilsImpl for timeline records based on Timeline agent events
This patch moves some of UI-specific getters from TimelineModel.Record into TimelineUIUtils instance methods. In the end we should have all ui logic that depends on particular backend details to be encapsulated in TimelineUIUtils instance.

BUG=361045
R=caseq@chromium.org

Review URL: https://codereview.chromium.org/344443007

git-svn-id: svn://svn.chromium.org/blink/trunk@176405 bbb929c8-8fbe-4397-9dbb-9b2b20218538