Added a console warning when <picture><source src></picture> is used.

Source/core/frame/UseCounter.cpp

Index: Source/core/frame/UseCounter.cpp
diff --git a/Source/core/frame/UseCounter.cpp b/Source/core/frame/UseCounter.cpp
index 764140188a025fbf33ea623d50f39638dd01b2cc..40be00b421d5bdd55bf89c843eae686907f72d09 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -785,6 +785,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case HTMLTableElementHspace:
         return "The 'hspace' attribute on table is deprecated. Please use CSS instead.";
 
+    case PictureSourceSrc:
+        return "<source src> with a <picture> parent is invalid and therefore ignored. Please use <source srcset> instead.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();