ppapi_proxy: Support loading and reading urls. Proxy URLLoader....

src/shared/ppapi_proxy/completion_callback.srpc

 # Copyright (c) 2010 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # RPC method that supports invocation of plugin-side callbacks from the browser.
 {
  'name': 'CompletionCallbackRpc',
  'rpcs': [
           {'name': 'RunCompletionCallback',
            'inputs': [['callback_id', 'int32_t'],
                       ['result', 'int32_t'],
+                      ['read_buffer', 'char[]' ],
                      ],
            'outputs': []
           },
          ]
 }