Improvements to network control types.

This CL prepares for adding the BBR network controller and
unit tests for GoogCC network controller.

The changes include:
* Adding pad_rate helper method on PacerConfig.
* Adding ostream operators for controller feedback structs.
* Adding increment operator to Timestamp class.
* Adding kEpoch to Timestamp class to represent 0.
* Rounding when multiplying with double.

Bug: webrtc:8415
Change-Id: I58289f37a6f9f2eee0a88bb06fb24dc295942862
Reviewed-on: https://webrtc-review.googlesource.com/61503
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22458}

modules/congestion_controller/goog_cc/BUILD.gn

@@ -155,6 +155,7 @@ if (rtc_include_tests) {
       "../../remote_bitrate_estimator",
       "../../rtp_rtcp:rtp_rtcp_format",
       "../network_control",
+      "../network_control:network_control_test",
       "//testing/gmock",
     ]
     if (!build_with_chromium && is_clang) {

modules/congestion_controller/goog_cc/probe_controller_unittest.cc

@@ -11,6 +11,7 @@
 
 #include "modules/congestion_controller/goog_cc/probe_controller.h"
 #include "modules/congestion_controller/network_control/include/network_types.h"
+#include "modules/congestion_controller/network_control/test/network_ostream_operators.h"
 #include "rtc_base/logging.h"
 #include "system_wrappers/include/clock.h"
 #include "test/gmock.h"