Mozart: Move frame lag compensation to new class.

mojo/services/gfx/composition/interfaces/scheduling.mojom

 // Copyright 2015 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.
 
 [DartPackage="mojo_services"]
 module mojo.gfx.composition;
 
 // Provides support for scheduling drawing and composition operations
 // for a particular scene.
 //
@@ skipping 24 matching lines @@
   // reported information, it's still better for them to schedule each frame
   // individually so they stay in sync with one another and quickly catch
   // up to any changes in timing.  This also gives the compositor more
   // opportunity to plan for the upcoming frame, perhaps even boost clocks
   // in anticipation.
   ScheduleFrame() => (FrameInfo frame_info);
 };
 
 // Provides timestamp information about a frame which has been scheduled
 // to be drawn.
+//
+// As there may be latency in receiving an updated |FrameInfo| from the
+// system, the consumer of this structure should apply compensation for
+// skipped frames.
+//
+// See |mojo::gfx::composition::FrameTracker|.
 struct FrameInfo {
   // A timestamp indicating when the work of updating the frame was scheduled
   // to begin which may be used to coordinate animations that require a
   // monotonic timing reference for synchronization, even across scenes.
   //
   // It essentially represents the time when the work of producing a new
   // frame began.
   //
   // This value monotonically increases with each frame, never repeats, and
   // is guaranteed to represent a time in the recent past.  It will always be
@@ skipping 27 matching lines @@
   // A timestamp indicating approximately when the contents of the frame
   // will be shown on the display output assuming everything is fully rendered
   // and submitted by the indicated |frame_deadline|.
   //
   // This value monotonically increases with each frame, never repeats, and
   // is guaranteed to be no less than |frame_deadline|.
   //
   // Expressed in microseconds in the |MojoTimeTicks| timebase.
   int64 presentation_time;
 };