cc: Update SpreadForStdDeviation to 3*sigma, and update tests.

cc/output/filter_operations.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 "cc/output/filter_operations.h"
 
 #include <stddef.h>
 
 #include <cmath>
 #include <numeric>
@@ skipping 33 matching lines @@
 
 void FilterOperations::Clear() {
   operations_.clear();
 }
 
 bool FilterOperations::IsEmpty() const {
   return operations_.empty();
 }
 
 static int SpreadForStdDeviation(float std_deviation) {
-  // https://dvcs.w3.org/hg/FXTF/raw-file/tip/filters/index.html#feGaussianBlurElement
-  // provides this approximation for evaluating a gaussian blur by a triple box
-  // filter.
-  float d = floorf(std_deviation * 3.f * sqrt(8.f * atan(1.f)) / 4.f + 0.5f);
-  return static_cast<int>(ceilf(d * 3.f / 2.f));
+  // Corresponds to MapStdDeviation in filter_operation.cc.
+  return std_deviation * 3;
 }
 
 gfx::Rect FilterOperations::MapRect(const gfx::Rect& rect,
                                     const SkMatrix& matrix) const {
   auto accumulate_rect = [matrix](const gfx::Rect& rect,
                                   const FilterOperation& op) {
     return op.MapRect(rect, matrix);
   };
   return std::accumulate(operations_.begin(), operations_.end(), rect,
                          accumulate_rect);
@@ skipping 155 matching lines @@
 void FilterOperations::AsValueInto(
     base::trace_event::TracedValue* value) const {
   for (size_t i = 0; i < operations_.size(); ++i) {
     value->BeginDictionary();
     operations_[i].AsValueInto(value);
     value->EndDictionary();
   }
 }
 
 }  // namespace cc