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

cc/output/filter_operations.cc

Index: cc/output/filter_operations.cc
diff --git a/cc/output/filter_operations.cc b/cc/output/filter_operations.cc
index c5f18dec8d6557f677e8974fd8d5c56de5947585..00ec79f52c24c0d0576b94afff50cf4015b6d8d9 100644
--- a/cc/output/filter_operations.cc
+++ b/cc/output/filter_operations.cc
@@ -51,11 +51,8 @@ bool FilterOperations::IsEmpty() const {
 }
 
 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,