nsIStreamListener in Firefox 3.5

 Called when the next chunk of data (corresponding to the request) may
 be read without blocking the calling thread.  The onDataAvailable impl
 must read exactly |aCount| bytes of data before returning.

 @param aRequest request corresponding to the source of the data
 @param aContext user defined context
 @param aInputStream input stream containing the data chunk
 @param aOffset current stream position
 @param aCount number of bytes available in the stream

 NOTE: The aInputStream parameter must implement readSegments.

 An exception thrown from onDataAvailable has the side-effect of
 causing the request to be canceled.