Reduced the callers to the non-const version of CSSValueList::item()

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

Index: third_party/WebKit/Source/core/css/CSSImageSetValue.cpp
diff --git a/third_party/WebKit/Source/core/css/CSSImageSetValue.cpp b/third_party/WebKit/Source/core/css/CSSImageSetValue.cpp
index a294539c34fa60d250ce97511633d137319f7fa1..d54a9b97636f946874110c94d4867364faa5e0ef 100644
--- a/third_party/WebKit/Source/core/css/CSSImageSetValue.cpp
+++ b/third_party/WebKit/Source/core/css/CSSImageSetValue.cpp
@@ -57,12 +57,12 @@ void CSSImageSetValue::fillImageSet()
     size_t length = this->length();
     size_t i = 0;
     while (i < length) {
-        CSSImageValue* imageValue = toCSSImageValue(item(i));
+        const CSSImageValue* imageValue = toCSSImageValue(item(i));
         String imageURL = imageValue->url();
 
         ++i;
         ASSERT_WITH_SECURITY_IMPLICATION(i < length);
-        CSSValue* scaleFactorValue = item(i);
+        const CSSValue* scaleFactorValue = item(i);
         float scaleFactor = toCSSPrimitiveValue(scaleFactorValue)->getFloatValue();
 
         ImageWithScale image;