[Fetch] Add a deprecation warning for Request.context

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 0d25a35e41dcdc153889d3a091f700506aac8179..7dc514c3e075c9cce4b4408c827284b2fe3da452 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -968,6 +968,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case PrefixedPerformanceResourceTimingBufferFull:
         return replacedBy("Performance.onwebkitresourcetimingbufferfull", "Performance.onresourcetimingbufferfull");
 
+    case FetchAPIRequestContext:
+        return "Request.context is deprecated and will be removed in M46 (see: https://www.chromestatus.com/feature/5534702526005248).";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();