Timeline: remove redundant header from frame statistics

- rename generatePopupContent.* to generateDetailsContent where appropriate;
- change generateDetailsContentForFrame to use TimelineDetailsContentHelper,
    thus removing redundant header;
- (drive-by) fix frame start time in case frames are built from trace events.

Review URL: https://codereview.chromium.org/341483005

git-svn-id: svn://svn.chromium.org/blink/trunk@176397 bbb929c8-8fbe-4397-9dbb-9b2b20218538