MacViews: exit early when trying to drag without a drag image instead of crashing on Mojave

This is a temporary patch to merge back to 69 while we figure out the root cause.

Bug: 876201
Change-Id: Id87ac81f16b1cd3102c8605691421e8bc53dc401
Reviewed-on: https://chromium-review.googlesource.com/1183991Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585001}

ui/views/cocoa/drag_drop_client_mac.mm

@@ -104,6 +104,12 @@ void DragDropClientMac::StartDragAndDrop(
   NSImage* image = gfx::NSImageFromImageSkiaWithColorSpace(
       provider.GetDragImage(), base::mac::GetSRGBColorSpace());
 
+  // TODO(crbug/876201): This shouldn't happen. When a repro for this
+  // is identified and the bug is fixed, change the early return to
+  // a DCHECK.
+  if (!image || NSEqualSizes([image size], NSZeroSize))
+    return;
+
   base::scoped_nsobject<NSPasteboardItem> item([[NSPasteboardItem alloc] init]);
   [item setDataProvider:data_source_.get()
                forTypes:provider.GetAvailableTypes()];