Ignore drag/drop input events in Mac

Makes drag/drop events obey the "ignore events" flag on WebContents.
Also add extra early returns in "update" and "exit" since those can
be called if the user drags/drops right after a scan completed.

This CL is the Mac version of the changes submitted in
crrev.com/c/2078654 for Aura.

Bug: 1045015
Change-Id: I4dfa3b2675b344ce7befa6be37f24d89957bf16e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078782Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745422}

content/browser/web_contents/web_drag_dest_mac.mm

@@ -178,6 +178,9 @@ void DropCompletionCallback(
 }
 
 - (NSDragOperation)draggingEntered:(const DraggingInfo*)info {
+  if (_webContents->ShouldIgnoreInputEvents())
+    return NSDragOperationNone;
+
   // Save off the RVH so we can tell if it changes during a drag. If it does,
   // we need to send a new enter message in draggingUpdated:.
   _currentRVH = _webContents->GetRenderViewHost();
@@ -239,6 +242,12 @@ void DropCompletionCallback(
 }
 
 - (void)draggingExited {
+  if (_webContents->ShouldIgnoreInputEvents())
+    return;
+
+  if (!_dropDataFiltered || !_dropDataUnfiltered)
+    return;
+
   DCHECK(_currentRVH);
   if (_currentRVH != _webContents->GetRenderViewHost())
     return;
@@ -261,6 +270,12 @@ void DropCompletionCallback(
 }
 
 - (NSDragOperation)draggingUpdated:(const DraggingInfo*)info {
+  if (_webContents->ShouldIgnoreInputEvents())
+    return NSDragOperationNone;
+
+  if (!_dropDataFiltered || !_dropDataUnfiltered)
+    return NSDragOperationNone;
+
   if (_canceled) {
     // TODO(ekaramad,paulmeyer): We probably shouldn't be checking for
     // |canceled_| twice in this method.
@@ -320,6 +335,9 @@ void DropCompletionCallback(
 - (BOOL)performDragOperation:(const DraggingInfo*)info
     withWebContentsViewDelegate:
         (content::WebContentsViewDelegate*)webContentsViewDelegate {
+  if (_webContents->ShouldIgnoreInputEvents())
+    return NO;
+
   gfx::PointF transformedPt;
   content::RenderWidgetHostImpl* targetRWH =
       [self GetRenderWidgetHostAtPoint:info->location_in_view