Fix EditingStyle::mergeStyle()'s handling of custom properties

third_party/WebKit/Source/core/css/StylePropertySet.cpp

Index: third_party/WebKit/Source/core/css/StylePropertySet.cpp
diff --git a/third_party/WebKit/Source/core/css/StylePropertySet.cpp b/third_party/WebKit/Source/core/css/StylePropertySet.cpp
index 1671630d5247e427093e6305b8a3dd9da4108092..a3bcaea4e3a7eee79e71589df7c046f835ae523e 100644
--- a/third_party/WebKit/Source/core/css/StylePropertySet.cpp
+++ b/third_party/WebKit/Source/core/css/StylePropertySet.cpp
@@ -290,6 +290,8 @@ bool StylePropertySet::isPropertyImplicit(CSSPropertyID propertyID) const
 
 bool MutableStylePropertySet::setProperty(CSSPropertyID unresolvedProperty, const String& value, bool important, StyleSheetContents* contextStyleSheet)
 {
+    DCHECK_GE(unresolvedProperty, firstCSSProperty);
+
     // Setting the value to an empty string just removes the property in both IE and Gecko.
     // Setting it to null seems to produce less consistent results, but we treat it just the same.
     if (value.isEmpty())