Add HpackDecoder3, an adapter using HpackDecoder (in net/http2/hpack/decoder).

net/spdy/hpack/hpack_decoder_interface.h

Index: net/spdy/hpack/hpack_decoder_interface.h
diff --git a/net/spdy/hpack/hpack_decoder_interface.h b/net/spdy/hpack/hpack_decoder_interface.h
index 8adb62ffd97a330f89a55c6cb1d8151499633bb0..1103e3198fb0b1d2b0205eb3fbfb131c298122ad 100644
--- a/net/spdy/hpack/hpack_decoder_interface.h
+++ b/net/spdy/hpack/hpack_decoder_interface.h
@@ -28,11 +28,13 @@ class NET_EXPORT_PRIVATE HpackDecoderInterface {
 
   // If a SpdyHeadersHandlerInterface is provided, the decoder will emit
   // headers to it rather than accumulating them in a SpdyHeaderBlock.
+  // Does not take ownership of the handler, but does use the pointer until
+  // the current HPACK block is completely decoded.
   virtual void HandleControlFrameHeadersStart(
       SpdyHeadersHandlerInterface* handler) = 0;
 
   // Called as HPACK block fragments arrive. Returns false if an error occurred
-  // while decoding the block.
+  // while decoding the block. Does not take ownership of headers_data.
   virtual bool HandleControlFrameHeadersData(const char* headers_data,
                                              size_t headers_data_length) = 0;
 
@@ -57,6 +59,11 @@ class NET_EXPORT_PRIVATE HpackDecoderInterface {
       std::unique_ptr<HpackHeaderTable::DebugVisitorInterface> visitor) = 0;
 
   // Set how much encoded data this decoder is willing to buffer.
+  // TODO(jamessynge): Resolve definition of this value, as it is currently
+  // too tied to a single implementation. We probably want to limit one or more
+  // of these: individual name or value strings, header entries, the entire
+  // header list, or the HPACK block; we probably shouldn't care about the size
+  // of individual transport buffers.
   virtual void set_max_decode_buffer_size_bytes(
       size_t max_decode_buffer_size_bytes) = 0;
 };