Enable transformed rasterization for kScrollDependentPosition

BUG=1076257
TEST=CompositedLayerMappingTest.TransformedRasterizationForScrollDependentPosition

Change-Id: I1ec70d65a533b14e3b6911de29830cbe334c66b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215335
Commit-Queue: Manuel Rego <rego@igalia.com>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775040}

third_party/blink/renderer/core/paint/compositing/composited_layer_mapping.cc

@@ -359,7 +359,7 @@ void CompositedLayerMapping::UpdateContentsOpaque() {
 void CompositedLayerMapping::UpdateRasterizationPolicy() {
   bool transformed_rasterization_allowed =
       !(owning_layer_.GetCompositingReasons() &
-        CompositingReason::kComboAllDirectReasons);
+        CompositingReason::kComboTransformedRasterizationDisallowedReasons);
   graphics_layer_->CcLayer()->SetTransformedRasterizationAllowed(
       transformed_rasterization_allowed);
   if (non_scrolling_squashing_layer_) {

third_party/blink/renderer/core/paint/compositing/composited_layer_mapping_test.cc

@@ -1394,6 +1394,29 @@ TEST_F(CompositedLayerMappingTest, TransformedRasterizationForInlineTransform) {
       target_graphics_layer->CcLayer()->transformed_rasterization_allowed());
 }
 
+TEST_F(CompositedLayerMappingTest,
+       TransformedRasterizationForScrollDependentPosition) {
+  SetBodyInnerHTML(R"HTML(
+    <div id="target"
+      style="transform: translateX(0.3px);
+             position: fixed; top: 20px; left: 30px;">
+        FIXED
+    </div>
+    <div style="height: 4000px; width: 4000px;
+         background: silver;">
+    </div>
+  )HTML");
+
+  LayoutObject* target = GetLayoutObjectByElementId("target");
+  ASSERT_TRUE(target && target->IsBox());
+  PaintLayer* target_layer = ToLayoutBox(target)->Layer();
+  GraphicsLayer* target_graphics_layer =
+      target_layer ? target_layer->GraphicsLayerBacking() : nullptr;
+  ASSERT_TRUE(target_graphics_layer);
+  EXPECT_TRUE(
+      target_graphics_layer->CcLayer()->transformed_rasterization_allowed());
+}
+
 TEST_F(CompositedLayerMappingTest, ScrollingContainerBoundsChange) {
   GetDocument().GetFrame()->GetSettings()->SetPreferCompositingToLCDTextEnabled(
       true);

third_party/blink/renderer/platform/graphics/compositing_reasons.h

@@ -132,6 +132,9 @@ class PLATFORM_EXPORT CompositingReason {
     kComboAllDirectReasons = kComboAllDirectStyleDeterminedReasons |
                              kComboAllDirectNonStyleDeterminedReasons,
 
+    kComboTransformedRasterizationDisallowedReasons =
+        kComboAllDirectReasons & ~kScrollDependentPosition,
+
     kComboAllCompositedScrollingDeterminedReasons =
         kScrollDependentPosition | kOverflowScrolling,