QUIC - increase buffer size to improve upload performance.

Increase the size of QuicHttpStream::raw_request_body_buf_ based on the
size of the body.  For large bodies and fast networks, this give a nice
reduction in CPU (or boost throughput if CPU bound) (~10-20%).

R=rch@chromium.org

Bug: 709144
Change-Id: If4fc69f9f89866b75b6d09d728e53a7bfa2054e4
Reviewed-on: https://chromium-review.googlesource.com/581877Reviewed-by: Buck Krasic <ckrasic@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Buck Krasic <ckrasic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488765}

net/quic/chromium/quic_http_stream.cc

@@ -222,10 +222,12 @@ int QuicHttpStream::SendRequest(const HttpRequestHeaders& request_headers,
     // was being called even if we didn't yet allocate raw_request_body_buf_.
     //   && (request_body_stream_->size() ||
     //       request_body_stream_->is_chunked()))
-    // Use 10 packets as the body buffer size to give enough space to
-    // help ensure we don't often send out partial packets.
-    raw_request_body_buf_ =
-        new IOBufferWithSize(static_cast<size_t>(10 * kMaxPacketSize));
+    // Set the body buffer size to be the size of the body clamped
+    // into the range [10 * kMaxPacketSize, 256 * kMaxPacketSize].
+    // With larger bodies, larger buffers reduce CPU usage.
+    raw_request_body_buf_ = new IOBufferWithSize(static_cast<size_t>(std::max(
+        10 * kMaxPacketSize,
+        std::min(request_body_stream_->size(), 256 * kMaxPacketSize))));
     // The request body buffer is empty at first.
     request_body_buf_ = new DrainableIOBuffer(raw_request_body_buf_.get(), 0);
   }