Implement responsiveness::MessageLoopObserver.

This CL adds the class MessageLoopObserver, which forwards events from the UI
and IO thread message loop task runners to the responsiveness Watcher and
Calculator.

Bug: 859155
Change-Id: Ie492f1fbc26940007cb303bdc5cdbdb59d887c19
Reviewed-on: https://chromium-review.googlesource.com/1149042
Commit-Queue: Erik Chen <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Reviewed-by: Timothy Dresser <[email protected]>
Cr-Commit-Position: refs/heads/master@{#579083}
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index 11cc745c..9f20f83 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -1518,6 +1518,8 @@
     "sandbox_parameters_mac.mm",
     "scheduler/responsiveness/calculator.cc",
     "scheduler/responsiveness/calculator.h",
+    "scheduler/responsiveness/message_loop_observer.cc",
+    "scheduler/responsiveness/message_loop_observer.h",
     "scheduler/responsiveness/watcher.cc",
     "scheduler/responsiveness/watcher.h",
     "scoped_active_url.cc",