[Tracing Clock Sync] Add TracingAgent interface in Chrome

This CL adds TracingAgent interface in Chrome and hooks up
corresponding subclasses.

Design doc: https://goo.gl/8Vy6qX

BUG=542390

Review URL: https://codereview.chromium.org/1468173003

Cr-Original-Commit-Position: refs/heads/master@{#365284}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b0b2a7f0fd7f6ec4a43f735e643a1f2f90351dc8

tools/gn/bootstrap/bootstrap.py

diff --git a/tools/gn/bootstrap/bootstrap.py b/tools/gn/bootstrap/bootstrap.py
index 5296ff3..6a6d12d 100755
--- a/tools/gn/bootstrap/bootstrap.py
+++ b/tools/gn/bootstrap/bootstrap.py
@@ -267,6 +267,7 @@
       'base/trace_event/trace_log.cc',
       'base/trace_event/trace_log_constants.cc',
       'base/trace_event/trace_sampling_thread.cc',
+      'base/trace_event/tracing_agent.cc',
       'base/tracked_objects.cc',
       'base/tracking_info.cc',
       'base/values.cc',