Bug 1271883 - Add Widevine keysystem to sGMPCodecs. r=jwwang

I broke Widevine in 1271242 when I changed EMEDecoderModule::SupportsMimeType()
to call GMPDecoderModule::SupportsMimeType() specifying a GMP to use, as I
didn't add the widevine keysystem to the take of keysystems that we recognise
in GMPDecoderModule. So we're unable to create an EME MediaDataDecoder.

In order to make Widevine EME work again, we need the Widevine keysystem string
in sGMPCodecs in GMPDecoderModule.cpp.

MozReview-Commit-ID: 6bJwsoGYIyL

--HG--
extra : rebase_source : be42b1848f10e2f4414319ffa8fe507d46e88cbc

dom/media/platforms/agnostic/gmp/GMPDecoderModule.cpp

@@ -156,6 +156,7 @@ struct GMPCodecs {
 static GMPCodecs sGMPCodecs[] = {
   { "org.w3.clearkey", false, false },
   { "com.adobe.primetime", false, false },
+  { "com.widevine.alpha", false, false },
 };
 
 void