GitLab

- marked most callbacks as optional;
- merged PanelWithSidebars into ElementsPanel;
- more specific description for most of the callbacks;
- misc puntuation fixes;^H.

BUG=none
TEST=none


Review URL: http://codereview.chromium.org/8538005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110485 0039d316-1c4b-4281-b951-d872f2087c98