Ensure the scheduler always changes from UseCase LOADING to NONE.

third_party/WebKit/Source/core/paint/FirstMeaningfulPaintDetector.cpp

 // Copyright 2016 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.
 
 #include "core/paint/FirstMeaningfulPaintDetector.h"
 
 #include "core/css/FontFaceSet.h"
 #include "core/dom/TaskRunnerHelper.h"
 #include "core/paint/PaintTiming.h"
 #include "platform/instrumentation/tracing/TraceEvent.h"
@@ skipping 105 matching lines @@
                                     BLINK_FROM_HERE);
 }
 
 void FirstMeaningfulPaintDetector::networkStableTimerFired(TimerBase*) {
   if (m_state == Reported || !document() ||
       document()->fetcher()->hasPendingRequest() ||
       !m_paintTiming->firstContentfulPaint())
     return;
 
   if (m_provisionalFirstMeaningfulPaint) {
+    // If there's only been one contentful paint, then there won't have been
+    // a meaningful paint signalled to the Scheduler, so mark one now.
+    // This is a no-op if a FMPC has already been marked.
+    m_paintTiming->markFirstMeaningfulPaintCandidate();

[kouhei (in TOK), 2017/03/13 19:22:48]

I'm convinced that we need to signal scheduler here, but I'm not sure if we
should mark PaintTiming::m_firstMeaningfulPaintCandidate.
delegating review to ksakamoto@

[Kunihiko Sakamoto, 2017/03/14 01:44:26]

Yeah, this would set a non-paint timestamp to
PaintTiming::m_firstMeaningfulPaintCandidate.

Instead, would you change

  PaintTiming::markFirstMeaningfulPaintCandidate() {
     ...
     m_firstMeaningfulPaintCandidate = monotonicallyIncreasingTime();
     ...
  }

to

  PaintTiming::setFirstMeaningfulPaintCandidate(double timestamp) {
     ...
     m_firstMeaningfulPaintCandidate = timestamp;
     ...
  }

and call it with m_provisionalFirstMeaningfulPaint?

     // Enforce FirstContentfulPaint <= FirstMeaningfulPaint.
     double timestamp = std::max(m_provisionalFirstMeaningfulPaint,
                                 m_paintTiming->firstContentfulPaint());
     m_paintTiming->setFirstMeaningfulPaint(timestamp);
   }
   m_state = Reported;
 }
 
 DEFINE_TRACE(FirstMeaningfulPaintDetector) {
   visitor->trace(m_paintTiming);
 }
 
 }  // namespace blink