Update the deprecation message for prefixed EME APIs.

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 5af04c876805c118e2e842409bd92a300c14df7b..c870b002a983314c39ca3b81f51236891aa39459 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -866,10 +866,10 @@ String UseCounter::deprecationMessage(Feature feature)
     case PrefixedMediaAddKey:
     case PrefixedMediaGenerateKeyRequest:
     case PrefixedMediaCancelKeyRequest:
-        return "The prefixed Encrypted Media Extensions APIs are deprecated and will be removed soon. Please use 'navigator.requestMediaKeySystemAccess()' instead.";
+        return "The prefixed Encrypted Media Extensions APIs are deprecated and will be removed in M46 (beta around September 2015). Please use 'navigator.requestMediaKeySystemAccess()' instead.";
 
     case CanPlayTypeKeySystem:
-        return "canPlayType()'s 'keySystem' parameter is deprecated and will be removed soon. Please use 'navigator.requestMediaKeySystemAccess()' instead.";
+        return "canPlayType()'s 'keySystem' parameter is deprecated and will be ignored in M46 (beta around September 2015). Please use 'navigator.requestMediaKeySystemAccess()' instead.";
 
     case SVGSVGElementForceRedraw:
         return "'SVGSVGElement.forceRedraw()' is deprecated, please do not use it. It is a no-op, as per SVG2 (https://svgwg.org/svg2-draft/struct.html#__svg__SVGSVGElement__forceRedraw).";