Fixes <http://webkit.org/b/30315>.

Fixes <http://webkit.org/b/30315>.
Web Inspector: DOM Content and Load lines in Resources are unclear what they mean.

Reviewed by Timothy Hatcher.

Add tooltip text to the Load event and DOM Content event lines through
a new layer that allows these lines to be on top of the view.

* inspector/front-end/ResourcesPanel.js:
(WebInspector.ResourcesPanel.prototype._updateGraphDividersIfNeeded):
(WebInspector.ResourcesPanel.prototype._updateDividersLabelBarPosition):
* inspector/front-end/inspector.css:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@49479 268f45cc-cd09-0410-ab3c-d52691b4dbfc