Use SizesAttributeParser to get the right srcset resource

Source/core/css/parser/SizesAttributeParserTest.cpp

Index: Source/core/css/parser/SizesAttributeParserTest.cpp
diff --git a/Source/core/css/parser/SizesAttributeParserTest.cpp b/Source/core/css/parser/SizesAttributeParserTest.cpp
index 7a01053c72e039d460be9d5efbccc873696eebf3..7989722e8099ac089ee81f04da82551e60b5734f 100644
--- a/Source/core/css/parser/SizesAttributeParserTest.cpp
+++ b/Source/core/css/parser/SizesAttributeParserTest.cpp
@@ -46,6 +46,7 @@ TEST(SizesAttributeParserTest, Basic)
         {"asdf, 200px", 200},
         {"(max-width: 3000px) 200w, 400w", 500},
         {",, , /**/ ,200px", 200},
+        {"50vw", 250},
         // FIXME - test all other units, zero length and calc().
         {0, 0} // Do not remove the terminator line.
     };