Web Inspector: OverviewGrid. Dragged window may change its width due to accumulating rounding error.
https://bugs.webkit.org/show_bug.cgi?id=113138

Reviewed by Pavel Feldman.

The old version had problem with rounding because it recalculates the window size on each event.
The new version just calculates the dragging delta and moves the window
to the new position based on the initial values and the delta.

* inspector/front-end/OverviewGrid.js:
(WebInspector.OverviewGrid.Window.prototype._startWindowDragging):
(WebInspector.OverviewGrid.Window.prototype._windowDragging):
(WebInspector.OverviewGrid.Window.prototype._onMouseWheel):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146866 268f45cc-cd09-0410-ab3c-d52691b4dbfc