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To add the functionality of a reverse proxy to the QUIC server, 2 classes
were added:
1. QuicHttpProxyBackend: Implements the interface: QuicSimpleServerBackend
to fetch the response from a backend server. Creates a proxy thread and manages an instance of
   net::URLRequestContext within that thread to make HTTP calls to a backend
   server.
2. QuicHttpProxyBackendStream: Created on a per-stream basis, manages an
   instance of the class net::URLRequest to make a single HTTP call to the
   backend server using the context created by QuicHttpProxyBackend.

Run As a Proxy
To run the quic_server as a reverse proxy, run
with --mode=proxy. The param --quic_proxy_backend_url specifies the
backend server from which the response is fetched. For instance,

./out/Default/quic_server \
  --certificate_file=net/tools/quic/certs/out/leaf_cert.pem \
  --key_file=net/tools/quic/certs/out/leaf_cert.pkcs8 \
  --mode=proxy \
  --quic_proxy_backend_url=http://localhost

R=rch@chromium.org, zhongyi@chromium.org

Change-Id: I79b7d66eb6bba9628d99181def004c0a5ea2e214
Reviewed-on: https://chromium-review.googlesource.com/1128323
Commit-Queue: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577650}