TimelinePanel: move details view from TimelineView to TimelinePanel.

Later DetailsView might become a separate window.

BUG=326121
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/153833003

git-svn-id: svn://svn.chromium.org/blink/trunk@166409 bbb929c8-8fbe-4397-9dbb-9b2b20218538