Web Inspector: color wheel should support Retina displays
https://bugs.webkit.org/show_bug.cgi?id=124355

Reviewed by NOBODY (OOPS!).

Take into account the devicePixelRatio in order to draw the color wheel
with as many colors as we can draw with the current DPI and to correctly
convert from page coordinates to canvas coordinates when dealing with
mouse events.

* UserInterface/ColorWheel.js:
(WebInspector.ColorWheel.prototype.set dimension):
(WebInspector.ColorWheel.prototype.get tintedColor):
(WebInspector.ColorWheel.prototype.get rawColor):
(WebInspector.ColorWheel.prototype._drawRawCanvas):
(WebInspector.ColorWheel.prototype._colorAtPointWithBrightness):
(WebInspector.ColorWheel.prototype._drawTintedCanvas):
(WebInspector.ColorWheel.prototype._draw):