Bug 1562048 - Draw the fallback drag image using a different API. r=spohl

This also fixes a leak - the return value is expected to be autoreleased.

Differential Revision: https://phabricator.services.mozilla.com/D36252

--HG--
extra : moz-landing-system : lando

widget/cocoa/nsDragService.mm

@@ -72,18 +72,15 @@ NSImage* nsDragService::ConstructDragImage(nsINode* aDOMNode, const Maybe<CSSInt
     NSSize size;
     size.width = nsCocoaUtils::DevPixelsToCocoaPoints(dragRect.width, scaleFactor);
     size.height = nsCocoaUtils::DevPixelsToCocoaPoints(dragRect.height, scaleFactor);
-    image = [[NSImage alloc] initWithSize:size];
-    [image lockFocus];
-    [[NSColor grayColor] set];
-    NSBezierPath* path = [NSBezierPath bezierPath];
-    [path setLineWidth:2.0];
-    [path moveToPoint:NSMakePoint(0, 0)];
-    [path lineToPoint:NSMakePoint(0, size.height)];
-    [path lineToPoint:NSMakePoint(size.width, size.height)];
-    [path lineToPoint:NSMakePoint(size.width, 0)];
-    [path lineToPoint:NSMakePoint(0, 0)];
-    [path stroke];
-    [image unlockFocus];
+    image = [NSImage imageWithSize:size
+                           flipped:YES
+                    drawingHandler:^BOOL(NSRect dstRect) {
+                      [[NSColor grayColor] set];
+                      NSBezierPath* path = [NSBezierPath bezierPathWithRect:dstRect];
+                      [path setLineWidth:2.0];
+                      [path stroke];
+                      return YES;
+                    }];
   }
 
   LayoutDeviceIntPoint pt(dragRect.x, dragRect.YMost());