nsIStreamListener
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.
NOTE: The aInputStream parameter must implement readSegments.
An exception thrown from onDataAvailable has the side-effect of
causing the request to be canceled.
| aRequest | request corresponding to the source of the data |
| aContext | user defined context |
| aInputStream | input stream containing the data chunk |
| aOffset | Number of bytes that were sent in previous onDataAvailable calls for this request. In other words, the sum of all previous count parameters. |
| aCount | number of bytes available in the stream |