Cancel drag operation when the tab RenderWidgetHostView does not exist (Mac)

content/browser/web_contents/web_drag_dest_mac.mm

Index: content/browser/web_contents/web_drag_dest_mac.mm
diff --git a/content/browser/web_contents/web_drag_dest_mac.mm b/content/browser/web_contents/web_drag_dest_mac.mm
index 625df3f1ecf6d60aa7f1306ead3da079e8f30172..ff7abd20eeaf52069ab0c7e0ebcb9e7473be071c 100644
--- a/content/browser/web_contents/web_drag_dest_mac.mm
+++ b/content/browser/web_contents/web_drag_dest_mac.mm
@@ -134,6 +134,12 @@ int GetModifierFlags() {
   NSPoint viewPoint = [self flipWindowPointToView:windowPoint view:view];
   NSPoint screenPoint = [self flipWindowPointToScreen:windowPoint view:view];
   gfx::Point transformedPt;
+  if (!webContents_->GetRenderWidgetHostView()) {
+    // This could happen when the renderer process for the top-level RWH crashes
+    // (see https://crbug.com/670645).
+    canceled_ = true;
+    return NSDragOperationNone;
+  }
   currentRWHForDrag_ =
       [self GetRenderWidgetHostAtPoint:viewPoint transformedPt:&transformedPt]
           ->GetWeakPtr();
@@ -203,6 +209,9 @@ int GetModifierFlags() {
 
 - (NSDragOperation)draggingUpdated:(id<NSDraggingInfo>)info
                               view:(NSView*)view {
+  if (canceled_)
+    return NSDragOperationNone;

[dcheng, 2016/12/03 00:33:07]

Seems unusual that this condition is checked twice in this function.

[EhsanK, 2016/12/03 00:36:44]

I think draggingEntered could also mutate |canceled_|.

[dcheng, 2016/12/03 00:49:14]

Right, I understand, that was there even before we rewrote this to support
OOPIF.

It just feels weird: canceled_ originally represented whether or not the
delegate cancelled the drag. Now it means that, and "there is no RWHV", and it
feels like the two checks are entangled in a slightly unusual way.

[EhsanK, 2016/12/03 00:59:37]

I see your point. I could instead not change canceled_ but at the top verify if
|RWHV| exists.

[dcheng, 2016/12/03 01:02:32]

Meh, I can't really think of anything better atm. We need to clean this code up
anyway (we double hit-test in the draggingUpdated but not draggingEntered yet
case), we can figure it out then =)

[EhsanK, 2016/12/03 01:13:50]

I added TODOs (on behalf of paulmeyer@ as well hope that is alright).  I could
also remove this cancel and repeat verifying GetRenderWidgetHostView() ==
nullptr like above. WDYT?

+
   // Create the appropriate mouse locations for WebCore. The draggingLocation
   // is in window coordinates. Both need to be flipped.
   NSPoint windowPoint = [info draggingLocation];