Make decodeAudioData more precise. (WebAudio#1781)

* Make decodeAudioData more precise.

This explicitely refers to the algorithm to use in mimesniff, and
adjusts the algorithm structure to be able to not attempt to decode if
the mimesniff algorithm failed.

This fixes WebAudio#1563.

* Address review comments.

* address more review comments

index.bs

@@ -1132,12 +1132,21 @@ Methods</h4>
 			Note: Multiple <em>decoding threads</em> can run in parallel to
 			service multiple calls to <code>decodeAudioData</code>.
 
-			1. Attempt to decode the encoded {{BaseAudioContext/decodeAudioData(audioData, successCallback, errorCallback)/audioData!!argument}} into linear
-				PCM.
+			1. Let <var>can decode</var> be a boolean flag, initially set to true.
 
-			2. If a decoding error is encountered due to the audio format
-				not being recognized or supported, or because of
-				corrupted/unexpected/inconsistent data, then <a>queue a task</a> to
+			2. Attempt to determine the MIME type of
+				{{BaseAudioContext/decodeAudioData(audioData, successCallback,
+				errorCallback)/audioData!!argument}}, using
+				[[mimesniff#matching-an-audio-or-video-type-pattern]]. If the audio or
+				video type pattern matching algorithm returns <code>undefined</code>,
+				set <var>can decode</var> to <em>false</em>.
+
+			3. If <var>can decode</var> is <em>true</em>, attempt to decode the encoded
+				{{BaseAudioContext/decodeAudioData(audioData, successCallback,
+				errorCallback)/audioData!!argument}} into linear PCM. In case of
+				failure, set <var>can decode</var> to <em>false</em>.
+
+			4. If <var>can decode</var> is <em>false</em>, <a>queue a task</a> to
 				execute the following step, on the <a>control thread</a>'s
 				event loop:
 
@@ -1148,7 +1157,7 @@ Methods</h4>
 
 				3. If {{BaseAudioContext/decodeAudioData()/errorCallback!!argument}} is not missing, invoke {{BaseAudioContext/decodeAudioData()/errorCallback!!argument}} with <var>error</var>.
 
-			3. Otherwise:
+			5. Otherwise:
 				1. Take the result, representing the decoded linear PCM
 					audio data, and resample it to the sample-rate of the
 					{{AudioContext}} if it is different from