Implement "Key Presence" step in "Encrypted Block Encounted" algorithm in EME.

media/base/decryptor.h

Index: media/base/decryptor.h
diff --git a/media/base/decryptor.h b/media/base/decryptor.h
index 442c3c66ead98a01dc925df5a49f5a07c423d42e..56696695037a8ce1efba475b67552d6078bcabfe 100644
--- a/media/base/decryptor.h
+++ b/media/base/decryptor.h
@@ -34,7 +34,8 @@ class MEDIA_EXPORT Decryptor {
 
   enum DecryptStatus {
     kSuccess,  // Decryption successfully completed. Decrypted buffer ready.
-    kError  // Error occurred during decryption.
+    kNoKey,  // No key is available to decrypt.
+    kError  // Key is available but an error occurred during decryption.
   };
 
   Decryptor() {}
@@ -68,6 +69,8 @@ class MEDIA_EXPORT Decryptor {
   //
   // If the returned status is kSuccess, the |encrypted| buffer is successfully
   // decrypted and the decrypted buffer is ready to be read.
+  // If the returned status is kNoKey, no decryption key is available to decrypt
+  // |encrypted| buffer. In this case the decrypted buffer must be NULL.
   // If the returned status is kError, unexpected error has occurred. In this
   // case the decrypted buffer must be NULL.
   typedef base::Callback<void(DecryptStatus,