Issue #2073 - m-c 1383404: make SourceBuffer::Compact more efficient (squashed)

The first part also means that Compact no longer needs the SurfaceCache lock (used to be via CreateChunk->CanHold),
which avoids potential deadlocks during shutdown that m-c 523950 would otherwise cause

toolkit/components/places/PageIconProtocolHandler.js

@@ -92,6 +92,7 @@ PageIconProtocolHandler.prototype = {
 
         try {
           channel.contentType = mime;
+          channel.contentLength = len;
           // Pass the icon data to the output stream.
           let stream = Cc["@mozilla.org/binaryoutputstream;1"]
                          .createInstance(Ci.nsIBinaryOutputStream);