Web Animations CSS: Start running animations on the compositor

Source/core/css/CSSComputedStyleDeclaration.cpp

Index: Source/core/css/CSSComputedStyleDeclaration.cpp
diff --git a/Source/core/css/CSSComputedStyleDeclaration.cpp b/Source/core/css/CSSComputedStyleDeclaration.cpp
index 164c4a0f5161b28691e68dfdff265f690c0d53af..b4ef9fe2b0bb25150287407e8f0c7bbd70fc6c97 100644
--- a/Source/core/css/CSSComputedStyleDeclaration.cpp
+++ b/Source/core/css/CSSComputedStyleDeclaration.cpp
@@ -29,6 +29,9 @@
 #include "RuntimeEnabledFeatures.h"
 #include "StylePropertyShorthand.h"
 #include "bindings/v8/ExceptionState.h"
+#include "core/animation/ActiveAnimations.h"
+#include "core/animation/AnimationClock.h"
+#include "core/animation/DocumentTimeline.h"
 #include "core/css/BasicShapeFunctions.h"
 #include "core/css/CSSArrayFunctionValue.h"
 #include "core/css/CSSAspectRatioValue.h"
@@ -1620,6 +1623,16 @@ PassRefPtr<CSSValue> CSSComputedStyleDeclaration::getPropertyCSSValue(CSSPropert
     if (updateLayout) {
         Document& document = styledNode->document();
 
+        // If a compositor animation is running we may need to service animations
+        // in order to generate an up to date value.
+        if (RuntimeEnabledFeatures::webAnimationsCSSEnabled() && styledNode->isElementNode()) {
+            const Element* element = toElement(styledNode);
+            if (const ActiveAnimations* activeAnimations = element->activeAnimations()) {
+                if (activeAnimations->isRunningCompositorAnimation(propertyID))
+                    document.serviceAnimations(monotonicallyIncreasingTime());
+            }
+        }
+
         document.updateStyleForNodeIfNeeded(styledNode);
 
         // The style recalc could have caused the styled node to be discarded or replaced