Add Keyboard Latency UMA Metrics.

ui/latency/latency_info.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_LATENCY_LATENCY_INFO_H_
 #define UI_LATENCY_LATENCY_INFO_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 91 matching lines @@
   // but the swap failed.
   INPUT_EVENT_LATENCY_TERMINATED_SWAP_FAILED_COMPONENT,
   LATENCY_COMPONENT_TYPE_LAST =
       INPUT_EVENT_LATENCY_TERMINATED_SWAP_FAILED_COMPONENT,
 };
 
 enum SourceEventType {
   UNKNOWN,
   WHEEL,
   TOUCH,
+  // This only includes key presses, as they're the most representative of user
+  // perceived keyboard latency.
+  KEY,
   OTHER,
   SOURCE_EVENT_TYPE_LAST = OTHER,
 };
 
 class LatencyInfo {
  public:
   struct LatencyComponent {
     // Nondecreasing number that can be used to determine what events happened
     // in the component at the time this struct was sent on to the next
     // component.
@@ skipping 126 matching lines @@
 
 // This is declared here for use in gtest-based unit tests, but is defined in
 // //ui/latency:test_support target.
 // Without this the default PrintTo template in gtest tries to pass LatencyInfo
 // by value, which leads to an alignment compile error on Windows.
 void PrintTo(const LatencyInfo& latency, ::std::ostream* os);
 
 }  // namespace ui
 
 #endif  // UI_LATENCY_LATENCY_INFO_H_