Removing a DCHECK in OverscrollWindowAnimation::OnOverscrollModeChange

content/browser/web_contents/aura/overscroll_window_animation.cc

Index: content/browser/web_contents/aura/overscroll_window_animation.cc
diff --git a/content/browser/web_contents/aura/overscroll_window_animation.cc b/content/browser/web_contents/aura/overscroll_window_animation.cc
index 8ed863772f022b4b5f1e2d54008bbda2fc691bef..d3246764e2cfc8dd5024e3ad1ce95afb4dedc29f 100644
--- a/content/browser/web_contents/aura/overscroll_window_animation.cc
+++ b/content/browser/web_contents/aura/overscroll_window_animation.cc
@@ -121,7 +121,6 @@ void OverscrollWindowAnimation::OnOverscrollModeChange(
                                0);
   }
 
-  DCHECK_EQ(overscroll_source_, OverscrollSource::NONE);

[mohsen, 2017/07/20 17:22:37]

Do you mean that this is not always true? If yes, when/why?

[mfomitchev, 2017/07/20 17:28:09]

See the CL that I linked in the description. Basically it's possible for the
overlay to send OnOvercrollModeChange(NONE, WEST) via OverscrollWindowDelegate
while the "cancel" animation is in progress, then for the overlay to be
destroyed, and then for the OverscrollController to send the same
OnOvercrollModeChange. Then this DCHECK will trigger.

Obscure scenarios like this is one of the reasons it would be great to replace
this with a simplified version :)

[mohsen, 2017/07/20 18:14:26]

Opps! Sorry, I missed the CL in the description! 

   overscroll_source_ = source;
   slide_window_ = new_direction == SLIDE_FRONT
                       ? delegate_->CreateFrontWindow(slide_window_bounds)