Track plugin input event latency

ppapi/api/private/ppb_input_event_private.idl

+/* Copyright (c) 2014 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.
+ */
+
+/**
+ * This file defines the PPB_InputEvent_Private interfaces.
+ */
+
+label Chrome {
+  M36 = 0.1
+};
+
+/**
+ * The <code>PPB_InputEvent_Private</code> interface contains pointers to several
+ * functions related to generic input events on the browser.
+ */
+
+interface PPB_InputEvent_Private {
+  /**
+   * TraceInputLatency() traces the latency of the input event. The input
+   * latency is shown in the trace viewer. The starting point of the input
+   * latency is when the input event is sent from renderer to plugin. If the
+   * input event does not cause any rendering damage, the end point of input
+   * latency is when TraceInputLatency() is called on the event. If the input
+   * event does cause rendering damage, the end point of input latency is when
+   * the resulted plugin frame eventually reaches screen.

[dmichael (off chromium), 2014/05/05 22:20:32]

There are some caveats with this in your current patch:
1) Graphics3D is not part of it.
2) Graphics2D only works if it's bound. That makes sense, but there is a weird
case that could be surprising. If the plugin has 2 Graphics2Ds, one bound, and
one not...  the unbound one can steal the latency info objects, and never does
anything with them.

This is probably OK for a private interface, but probably worth documenting.

[Yufeng Shen (Slow to review), 2014/05/07 17:12:14]

Extra notes added.

+   *
+   * @param[in] event A <code>PP_Resource</code> corresponding to an input
+   * event.
+   *
+   * @param[in] has_damage A bool indicating whether the event has caused any
+   * rendering damage.
+   */
+
+  void TraceInputLatency([in] PP_Resource event,
+                         [in] PP_Bool has_damage);
+};
+