[Fetch] Add a deprecation warning for Request.context

Source/core/frame/UseCounter.cpp

 /*
  * Copyright (C) 2012 Google, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 950 matching lines @@
 
     case PrefixedPerformanceClearResourceTimings:
         return replacedBy("Performance.webkitClearResourceTimings", "Performance.clearResourceTimings");
 
     case PrefixedPerformanceSetResourceTimingBufferSize:
         return replacedBy("Performance.webkitSetResourceTimingBufferSize", "Performance.setResourceTimingBufferSize");
 
     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();
     }
 }
 
 void UseCounter::count(CSSParserContext context, CSSPropertyID feature)
 {
     ASSERT(feature >= firstCSSProperty);
     ASSERT(feature <= lastUnresolvedCSSProperty);
@@ skipping 27 matching lines @@
 UseCounter* UseCounter::getFrom(const StyleSheetContents* sheetContents)
 {
     // FIXME: We may want to handle stylesheets that have multiple owners
     //        http://crbug.com/242125
     if (sheetContents && sheetContents->hasSingleOwnerNode())
         return getFrom(sheetContents->singleOwnerDocument());
     return 0;
 }
 
 } // namespace blink