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

A recent change (https://codereview.chromium.org/1980613003) updated one path
to use 3*sigma as the estimated spread for gaussian blurs in filters. This
propagates that change to the "filter outset" path, for consistency, and
updates test results accordingly (including one visually-indistinguishable
pixel diff).

BUG=600821
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2004443002
Cr-Commit-Position: refs/heads/master@{#395126}

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);

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,

cc/output/filter_operations_unittest.cc

@@ -21,10 +21,10 @@ TEST(FilterOperationsTest, GetOutsetsBlur) {
   int top, right, bottom, left;
   top = right = bottom = left = 0;
   ops.GetOutsets(&top, &right, &bottom, &left);
-  EXPECT_EQ(57, top);
-  EXPECT_EQ(57, right);
-  EXPECT_EQ(57, bottom);
-  EXPECT_EQ(57, left);
+  EXPECT_EQ(60, top);
+  EXPECT_EQ(60, right);
+  EXPECT_EQ(60, bottom);
+  EXPECT_EQ(60, left);
 }
 
 TEST(FilterOperationsTest, MapRectBlur) {
@@ -100,10 +100,10 @@ TEST(FilterOperationsTest, GetOutsetsDropShadow) {
   int top, right, bottom, left;
   top = right = bottom = left = 0;
   ops.GetOutsets(&top, &right, &bottom, &left);
-  EXPECT_EQ(49, top);
-  EXPECT_EQ(60, right);
-  EXPECT_EQ(65, bottom);
-  EXPECT_EQ(54, left);
+  EXPECT_EQ(52, top);
+  EXPECT_EQ(63, right);
+  EXPECT_EQ(68, bottom);
+  EXPECT_EQ(57, left);
 }
 
 TEST(FilterOperationsTest, MapRectDropShadow) {

cc/trees/layer_tree_host_common_unittest.cc

@@ -1407,7 +1407,7 @@ TEST_F(LayerTreeHostCommonTest, RenderSurfaceListForFilter) {
   ASSERT_TRUE(parent->render_surface());
   EXPECT_EQ(2U, parent->render_surface()->layer_list().size());
   EXPECT_EQ(4U, render_surface_layer_list.size());
-  EXPECT_EQ(gfx::RectF(-29, -29, 158, 158),
+  EXPECT_EQ(gfx::RectF(-30, -30, 160, 160),
             parent->render_surface()->DrawableContentRect());
 }