TimelinePanel: convert a few getters on TimelineModel.Record into functions.

almost mechanical change. No business logic changes.

startTime, endTime, selfTime and category were converted.
Drive by fix: TimelineMergingRecordBuffer comparator types were specified incorrectly.

BUG=361045
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/256763002

git-svn-id: svn://svn.chromium.org/blink/trunk@172617 bbb929c8-8fbe-4397-9dbb-9b2b20218538