Make font-weight animatable.

Source/core/css/resolver/AnimatedStyleBuilder.cpp

Index: Source/core/css/resolver/AnimatedStyleBuilder.cpp
diff --git a/Source/core/css/resolver/AnimatedStyleBuilder.cpp b/Source/core/css/resolver/AnimatedStyleBuilder.cpp
index 6a27566d6e84d720d872d57de03ae601ac1d4d11..275460a84f64a251faeee610ad055e017b55cafb 100644
--- a/Source/core/css/resolver/AnimatedStyleBuilder.cpp
+++ b/Source/core/css/resolver/AnimatedStyleBuilder.cpp
@@ -225,6 +225,27 @@ void setOnFillLayers(FillLayer* fillLayer, const AnimatableValue* value, StyleRe
     }
 }
 
+FontWeight animatableValueToFontWeight(const AnimatableValue* value)
+{
+    int index = round(toAnimatableDouble(value)->toDouble() / 100) - 1;
+
+    static const FontWeight weights[] = {
+        FontWeight100,
+        FontWeight200,
+        FontWeight300,
+        FontWeight400,
+        FontWeight500,
+        FontWeight600,
+        FontWeight700,
+        FontWeight800,
+        FontWeight900
+    };
+
+    index = clampTo<int>(index, 0, WTF_ARRAY_LENGTH(weights) - 1);
+
+    return weights[index];
+}
+
 } // namespace
 
 // FIXME: Generate this function.
@@ -351,6 +372,9 @@ void AnimatedStyleBuilder::applyProperty(CSSPropertyID property, StyleResolverSt
     case CSSPropertyFontSize:
         style->setFontSize(clampTo<float>(toAnimatableDouble(value)->toDouble(), 0));
         return;
+    case CSSPropertyFontWeight:
+        style->setFontWeight(animatableValueToFontWeight(value));
+        return;
     case CSSPropertyHeight:
         style->setHeight(animatableValueToLength(value, state, NonNegativeValues));
         return;