Set the start bitrate to the delay-based BWE.

This avoids issues where the bitrate produced by the codec is far lower than the target bitrate in the beginning, which causes the delay-based BWE to be initialized accordingly.

BUG=webrtc:5079

Review-Url: https://codereview.webrtc.org/2653883002
Cr-Commit-Position: refs/heads/master@{#16327}

webrtc/modules/congestion_controller/congestion_controller_unittest.cc

@@ -240,6 +240,5 @@ TEST_F(CongestionControllerTest, OnReceivedPacketWithAbsSendTime) {
   ASSERT_EQ(1u, ssrcs.size());
   EXPECT_EQ(header.ssrc, ssrcs[0]);
 }
-
 }  // namespace test
 }  // namespace webrtc