Make it possible for the scroll offset delegate to intercept fling.

android_webview/browser/in_process_view_renderer.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "android_webview/browser/in_process_view_renderer.h"
 
 #include <android/bitmap.h>
 
 #include "android_webview/browser/scoped_app_gl_state_restore.h"
 #include "android_webview/public/browser/draw_gl.h"
@@ skipping 520 matching lines @@
   DCHECK(dip_scale_ > 0);
   // In general we don't guarantee that the scroll offset transforms are
   // symmetrical. That is if scrolling from JS to offset1 results in a native
   // offset2 then scrolling from UI to offset2 results in JS being scrolled to
   // offset1 again.
   // The reason we explicitly do rounding here is that it seems to yeld the
   // most stabile transformation.
   gfx::Vector2dF new_value_css = gfx::ToRoundedVector2d(
       gfx::ScaleVector2d(new_value, 1.0f / (dip_scale_ * page_scale_factor_)));
 
-  DCHECK(scroll_offset_css_ != new_value_css);
+  // It's possible that more than one set of unique physical coordinates maps
+  // to the same set of CSS coordinates which means we can't reliably early-out
+  // earlier in the call stack.
+  if (scroll_offset_css_ == new_value_css)
+    return;
 
   scroll_offset_css_ = new_value_css;
 
   if (compositor_)
     compositor_->DidChangeRootLayerScrollOffset();
 }
 
 void InProcessViewRenderer::DidUpdateContent() {
   if (on_new_picture_enable_)
     client_->OnNewPicture();
@@ skipping 117 matching lines @@
     base::StringAppendF(&str,
                         "surface width height: [%d %d] ",
                         draw_info->width,
                         draw_info->height);
     base::StringAppendF(&str, "is_layer: %d ", draw_info->is_layer);
   }
   return str;
 }
 
 }  // namespace android_webview