Add use counters for function values of easing

third_party/WebKit/Source/core/animation/AnimationInputHelpers.cpp

Index: third_party/WebKit/Source/core/animation/AnimationInputHelpers.cpp
diff --git a/third_party/WebKit/Source/core/animation/AnimationInputHelpers.cpp b/third_party/WebKit/Source/core/animation/AnimationInputHelpers.cpp
index 532cd58c4c39e71b101bb16206f6383603285b0a..21ad87c2d92a3d1c6933c3ad4e8f804440c820c2 100644
--- a/third_party/WebKit/Source/core/animation/AnimationInputHelpers.cpp
+++ b/third_party/WebKit/Source/core/animation/AnimationInputHelpers.cpp
@@ -202,7 +202,7 @@ const QualifiedName* AnimationInputHelpers::keyframeAttributeToSVGAttribute(cons
     return iter->value;
 }
 
-PassRefPtr<TimingFunction> AnimationInputHelpers::parseTimingFunction(const String& string)
+PassRefPtr<TimingFunction> AnimationInputHelpers::parseTimingFunction(const String& string, Document* document)
 {
     if (string.isEmpty())
         return nullptr;
@@ -210,6 +210,24 @@ PassRefPtr<TimingFunction> AnimationInputHelpers::parseTimingFunction(const Stri
     CSSValue* value = CSSParser::parseSingleValue(CSSPropertyTransitionTimingFunction, string);
     if (!value || !value->isValueList()) {
         ASSERT(!value || value->isCSSWideKeyword());
+        if (document) {
+            if (string.startsWith("function")) {
+                // Due to a bug in old versions of the web-animations-next
+                // polyfill, in some circumstances the string passed in here
+                // may be a Javascript function instead of the allowed values
+                // from the spec
+                // (http://w3c.github.io/web-animations/#dom-animationeffecttimingreadonly-easing)
+                // This bug was fixed in
+                // https://github.com/web-animations/web-animations-next/pull/423
+                // and we want to track how often it is still being hit. The
+                // linear case is special because 'linear' is the default value
+                // for easing.
+                if (string == "function (a){return a}")
+                    UseCounter::count(*document, UseCounter::WebAnimationsEasingAsFunctionLinear);
+                else
+                    UseCounter::count(*document, UseCounter::WebAnimationsEasingAsFunctionOther);
+            }
+        }
         return nullptr;
     }
     CSSValueList* valueList = toCSSValueList(value);