DevTools: Do not load the first panel appended to the panels tabbed pane

Show the first tab view lazily, when the tabbed pane is shown.

R=pfeldman@chromium.org, pfeldman, vsevik

Review URL: https://codereview.chromium.org/151903005

git-svn-id: svn://svn.chromium.org/blink/trunk@166357 bbb929c8-8fbe-4397-9dbb-9b2b20218538