[CompositeAfterPaint] Plumb the will-change: transform hint to cc::Layer

This patch plumbs the will-change: transform hint to cc::Layer if it is
a compositing reason on the immediate transform node.

Bug: 1031165
Change-Id: Ifc27d303d48fa957c8f0b4860058dfffabf70262
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1953340Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722546}

third_party/blink/renderer/core/exported/web_layer_test.cc

@@ -239,6 +239,23 @@ TEST_P(WebLayerListTest, FrameViewScroll) {
   EXPECT_EQ(ScrollOffset(0, 1), scrollable_area->GetScrollOffset());
 }
 
+TEST_P(WebLayerListTest, WillChangeTransformHint) {
+  InitializeWithHTML(*WebView()->MainFrameImpl()->GetFrame(), R"HTML(
+    <style>
+      #willChange {
+        width: 100px;
+        height: 100px;
+        will-change: transform;
+        background: blue;
+      }
+    </style>
+    <div id="willChange"></div>
+  )HTML");
+  UpdateAllLifecyclePhases();
+  auto* layer = ContentLayerAt(ContentLayerCount() - 1);
+  EXPECT_TRUE(layer->has_will_change_transform_hint());
+}
+
 class WebLayerListSimTest : public PaintTestConfigurations, public SimTest {
  public:
   void InitializeWithHTML(const String& html) {

third_party/blink/renderer/platform/graphics/compositing/paint_artifact_compositor.cc

@@ -1182,6 +1182,9 @@ void PaintArtifactCompositor::Update(
     bool backface_hidden = property_state.Transform().IsBackfaceHidden();
     layer->SetDoubleSided(!backface_hidden);
     layer->SetShouldCheckBackfaceVisibility(backface_hidden);
+    bool has_will_change_transform =
+        property_state.Transform().RequiresCompositingForWillChangeTransform();
+    layer->SetHasWillChangeTransformHint(has_will_change_transform);
 
     // If the property tree state has changed between the layer and the root,
     // we need to inform the compositor so damage can be calculated. Calling

third_party/blink/renderer/platform/graphics/paint/transform_paint_property_node.h

@@ -381,6 +381,11 @@ class PLATFORM_EXPORT TransformPaintPropertyNode
     return state_.direct_compositing_reasons & CompositingReason::kRootScroller;
   }
 
+  bool RequiresCompositingForWillChangeTransform() const {
+    return state_.direct_compositing_reasons &
+           CompositingReason::kWillChangeTransform;
+  }
+
   const CompositorElementId& GetCompositorElementId() const {
     return state_.compositor_element_id;
   }