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

cc/output/filter_operation.cc

Index: cc/output/filter_operation.cc
diff --git a/cc/output/filter_operation.cc b/cc/output/filter_operation.cc
index a9c52a2e80144d50916f3f3bf851968f40bbe36f..78588748ba59d63c5d68bd35e1df56cba76f0d22 100644
--- a/cc/output/filter_operation.cc
+++ b/cc/output/filter_operation.cc
@@ -316,6 +316,7 @@ void FilterOperation::AsValueInto(base::trace_event::TracedValue* value) const {
 }
 
 static SkVector MapStdDeviation(float std_deviation, const SkMatrix& matrix) {
+  // Corresponds to SpreadForStdDeviation in filter_operations.cc.
   SkVector sigma = SkVector::Make(std_deviation, std_deviation);
   matrix.mapVectors(&sigma, 1);
   return sigma * SkIntToScalar(3);