Ensure the scheduler always changes from UseCase LOADING to NONE.

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

Index: third_party/WebKit/Source/core/paint/FirstMeaningfulPaintDetector.cpp
diff --git a/third_party/WebKit/Source/core/paint/FirstMeaningfulPaintDetector.cpp b/third_party/WebKit/Source/core/paint/FirstMeaningfulPaintDetector.cpp
index 9fca4136977ee48f1cbc383fcf7596b21f0748de..802a148d34ee7be672adab8210727ab007d0178f 100644
--- a/third_party/WebKit/Source/core/paint/FirstMeaningfulPaintDetector.cpp
+++ b/third_party/WebKit/Source/core/paint/FirstMeaningfulPaintDetector.cpp
@@ -123,6 +123,10 @@ void FirstMeaningfulPaintDetector::networkStableTimerFired(TimerBase*) {
     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());