Web Inspector: add experiment for single click styles editing.

Web Inspector: add experiment for single click styles editing.
https://bugs.webkit.org/show_bug.cgi?id=77624

Reviewed by Vsevolod Vlasov.

* inspector/front-end/Settings.js:
(WebInspector.ExperimentsSettings):
* inspector/front-end/StylesSidebarPane.js:
(WebInspector.StylePropertiesSection):
(WebInspector.StylePropertyTreeElement.prototype.onattach):
(WebInspector.StylePropertyTreeElement.prototype._mouseDown):
(WebInspector.StylePropertyTreeElement.prototype._resetMouseDownElement):
(WebInspector.StylePropertyTreeElement.prototype):
(WebInspector.StylePropertyTreeElement.prototype.selectElement.context):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106550 268f45cc-cd09-0410-ab3c-d52691b4dbfc