Add back the truncation of scroll_delta

cc/trees/layer_tree_host_unittest_scroll.cc

 // Copyright 2012 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 "cc/trees/layer_tree_host.h"
 
 #include "base/memory/weak_ptr.h"
 #include "cc/layers/layer.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/picture_layer.h"
@@ skipping 411 matching lines @@
 
     // Check that a fractional scroll delta is correctly accumulated over
     // multiple commits.
     switch (impl->active_tree()->source_frame_number()) {
       case 0:
         EXPECT_VECTOR_EQ(scroll_layer->BaseScrollOffset(), gfx::Vector2d(0, 0));
         EXPECT_VECTOR_EQ(scroll_layer->ScrollDelta(), gfx::Vector2d(0, 0));
         PostSetNeedsCommitToMainThread();
         break;
       case 1:
-        EXPECT_VECTOR_EQ(scroll_layer->BaseScrollOffset(), scroll_amount_);
-        EXPECT_VECTOR_EQ(scroll_layer->ScrollDelta(), gfx::Vector2dF());
+        EXPECT_VECTOR_EQ(scroll_layer->BaseScrollOffset(),
+                         gfx::ToFlooredVector2d(scroll_amount_));
+        EXPECT_VECTOR_EQ(scroll_layer->ScrollDelta(),
+                         gfx::Vector2dF(fmod(scroll_amount_.x(), 1.0f), 0.0f));
         PostSetNeedsCommitToMainThread();
         break;
       case 2:
-        EXPECT_VECTOR_EQ(scroll_layer->BaseScrollOffset(),
-                         (scroll_amount_ + scroll_amount_));
-        EXPECT_VECTOR_EQ(scroll_layer->ScrollDelta(), gfx::Vector2dF());
+        EXPECT_VECTOR_EQ(
+            scroll_layer->BaseScrollOffset(),
+            gfx::ToFlooredVector2d(scroll_amount_ + scroll_amount_));
+        EXPECT_VECTOR_EQ(
+            scroll_layer->ScrollDelta(),
+            gfx::Vector2dF(fmod(2.0f * scroll_amount_.x(), 1.0f), 0.0f));
         EndTest();
         break;
     }
     scroll_layer->ScrollBy(scroll_amount_);
   }
 
   void AfterTest() override {}
 
  private:
   gfx::Vector2dF scroll_amount_;
@@ skipping 779 matching lines @@
   RunTest(true, false, true);
 }
 
 TEST_F(LayerTreeHostScrollTestLayerStructureChange, ScrollDestroyWholeTree) {
   scroll_destroy_whole_tree_ = true;
   RunTest(true, false, true);
 }
 
 }  // namespace
 }  // namespace cc