WIP: Plumbing input event latency reporting through Mus GPU.

ui/latency/BUILD.gn

Index: ui/latency/BUILD.gn
diff --git a/ui/latency/BUILD.gn b/ui/latency/BUILD.gn
index 5afbb031a17702a7a757367ece03950cbfb2cbe4..24e5a775a9ad22a2d842bb49ea47ddfa2c17f43a 100644
--- a/ui/latency/BUILD.gn
+++ b/ui/latency/BUILD.gn
@@ -6,16 +6,32 @@ import("//testing/test.gni")
 
 source_set("latency") {
   sources = [
+    "latency/latency_histogram_macros.h",
     "latency_info.cc",
     "latency_info.h",
+    "latency_tracker.cc",
+    "latency_tracker.h",
   ]
 
+  defines = [ "LATENCY_IMPLEMENTATION" ]
+
   deps = [
     "//base",
     "//ui/gfx",
   ]
 }
 
+source_set("test_support") {
+  testonly = true
+  sources = [
+    "latency_info_test_support.cc",
+  ]
+
+  public_deps = [
+    ":latency",
+  ]
+}
+
 test("latency_unittests") {
   sources = [
     "latency_info_unittest.cc",