DevTools: [Console] select property names with double click

As of today, double click on property name in ObjectPropertyTreeElement
results in editing of the property value. The patch changes the
behavior so that double click on property name just sets a selection,
and one has to double-click on property value to edit it.

BUG=423085
R=vsevik

Review URL: https://codereview.chromium.org/656123002

git-svn-id: svn://svn.chromium.org/blink/trunk@183760 bbb929c8-8fbe-4397-9dbb-9b2b20218538