bug 1205540 initialize mBufferFormat when constructing silent block r=padenot

This makes an AudioBlock valid for code testing mBufferFormat without IsNull(),
without the need for explicit SetNull().

This is useful so that setting AudioNodeStream::mLastChunks each iteration is
not required for inactive nodes.

--HG--
extra : rebase_source : 171b7339bfe91e15b070f03bd5e0b784b58a217a

dom/media/webaudio/AudioBlock.h

@@ -23,6 +23,7 @@ class AudioBlock : private AudioChunk
 public:
   AudioBlock() {
     mDuration = WEBAUDIO_BLOCK_SIZE;
+    mBufferFormat = AUDIO_FORMAT_SILENCE;
   }
   // No effort is made in constructors to ensure that mBufferIsDownstreamRef
   // is set because the block is expected to be a temporary and so the