nsIStreamListener Interface

 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
        Number of bytes that were sent in previous onDataAvailable calls
        for this request. In other words, the sum of all previous count
        parameters.
        If that number is greater than or equal to 2^32, this parameter
        will be PR_UINT32_MAX (2^32 - 1).
 @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.