Enable use of rtc::SystemTimeNanos() provided by Chromium

This is the third CL out of three to enable overriding
of the function SystemTimeNanos() in rtc_base/system_time.cc

When WebRTC is built as part of Chromium the rtc::SystemTimeNanos()
function provided by Chromium will be used. This is controlled
by the build argument rtc_exclude_system_time which directly
maps to the macro WEBRTC_EXCLUDE_SYSTEM_TIME.

By doing this we are making sure that the WebRTC and Chromium
clocks are the same.

Bug: chromium:516700
Change-Id: If7f749c4aadefb1cfc07ba4c7e3f45dc6c31118b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208223
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33337}

rtc_base/BUILD.gn

@@ -407,6 +407,10 @@ rtc_library("timeutils") {
     ":stringutils",
     "system:rtc_export",
   ]
+  if (rtc_exclude_system_time) {
+    defines = [ "WEBRTC_EXCLUDE_SYSTEM_TIME" ]
+  }
+
   libs = []
   if (is_win) {
     libs += [ "winmm.lib" ]

rtc_base/system_time.cc

@@ -8,6 +8,10 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+// If WEBRTC_EXCLUDE_SYSTEM_TIME is set, an implementation of
+// rtc::SystemTimeNanos() must be provided externally.
+#ifndef WEBRTC_EXCLUDE_SYSTEM_TIME
+
 #include <stdint.h>
 
 #include <limits>
@@ -90,3 +94,4 @@ int64_t SystemTimeNanos() {
 }
 
 }  // namespace rtc
+#endif  // WEBRTC_EXCLUDE_SYSTEM_TIME

rtc_tools/BUILD.gn

@@ -136,6 +136,12 @@ rtc_executable("frame_analyzer") {
     "//third_party/abseil-cpp/absl/flags:parse",
     "//third_party/abseil-cpp/absl/strings",
   ]
+
+  if (build_with_chromium) {
+    # When building from Chromium, WebRTC's metrics and field trial
+    # implementations need to be replaced by the Chromium ones.
+    deps += [ "//third_party/webrtc_overrides:webrtc_component" ]
+  }
 }
 
 # TODO(bugs.webrtc.org/11474): Enable this on win if needed. For now it

webrtc.gni

@@ -63,7 +63,7 @@ declare_args() {
   # will tell the pre-processor to remove the default definition of the
   # SystemTimeNanos() which is defined in rtc_base/system_time.cc. In
   # that case a new implementation needs to be provided.
-  rtc_exclude_system_time = false
+  rtc_exclude_system_time = build_with_chromium
 
   # Setting this to false will require the API user to pass in their own
   # SSLCertificateVerifier to verify the certificates presented from a