Bug 899430. Correct incorrect use GetDefaultScale in TextInputHandler to BackingScaleFactor. r=jfkthame

We are converting between cocoa points and dev pixels, cocoa points do not know about our prefs but GetDefaultScale takes into account our prefs, which is wrong.

widget/cocoa/TextInputHandler.mm

@@ -3073,7 +3073,7 @@ IMEInputHandler::FirstRectForCharacterRange(NSRange& aRange,
   if (!rootWindow || !rootView) {
     return rect;
   }
-  rect = nsCocoaUtils::DevPixelsToCocoaPoints(r, mWidget->GetDefaultScale());
+  rect = nsCocoaUtils::DevPixelsToCocoaPoints(r, mWidget->BackingScaleFactor());
   rect = [rootView convertRect:rect toView:nil];
   rect.origin = [rootWindow convertBaseToScreen:rect.origin];