[FirstMeaningfulPaint] Do not stop observing layouts before FirstContentfulPaint

This should mitigate a bug that FirstMeaningfulPaint is reported too
soon (or not reported) on some CPU-heavy pages, because network idle is
detected while CPU is busy running scripts.

BUG=659021,674228
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2581633002
Cr-Commit-Position: refs/heads/master@{#438791}

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

@@ -115,7 +115,8 @@ void FirstMeaningfulPaintDetector::checkNetworkStable() {
 
 void FirstMeaningfulPaintDetector::networkStableTimerFired(TimerBase*) {
   if (m_state == Reported || !document() ||
-      document()->fetcher()->hasPendingRequest())
+      document()->fetcher()->hasPendingRequest() ||
+      !m_paintTiming->firstContentfulPaint())
     return;
 
   if (m_provisionalFirstMeaningfulPaint)

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

@@ -59,7 +59,7 @@ TEST_F(FirstMeaningfulPaintDetectorTest, NoFirstPaint) {
 }
 
 TEST_F(FirstMeaningfulPaintDetectorTest, OneLayout) {
-  paintTiming().markFirstPaint();
+  paintTiming().markFirstContentfulPaint();
   simulateLayoutAndPaint(1);
   double afterPaint = monotonicallyIncreasingTime();
   EXPECT_EQ(paintTiming().firstMeaningfulPaint(), 0.0);
@@ -69,7 +69,7 @@ TEST_F(FirstMeaningfulPaintDetectorTest, OneLayout) {
 }
 
 TEST_F(FirstMeaningfulPaintDetectorTest, TwoLayoutsSignificantSecond) {
-  paintTiming().markFirstPaint();
+  paintTiming().markFirstContentfulPaint();
   simulateLayoutAndPaint(1);
   double afterLayout1 = monotonicallyIncreasingTime();
   simulateLayoutAndPaint(10);
@@ -80,7 +80,7 @@ TEST_F(FirstMeaningfulPaintDetectorTest, TwoLayoutsSignificantSecond) {
 }
 
 TEST_F(FirstMeaningfulPaintDetectorTest, TwoLayoutsSignificantFirst) {
-  paintTiming().markFirstPaint();
+  paintTiming().markFirstContentfulPaint();
   simulateLayoutAndPaint(10);
   double afterLayout1 = monotonicallyIncreasingTime();
   simulateLayoutAndPaint(1);
@@ -90,7 +90,7 @@ TEST_F(FirstMeaningfulPaintDetectorTest, TwoLayoutsSignificantFirst) {
 }
 
 TEST_F(FirstMeaningfulPaintDetectorTest, FirstMeaningfulPaintCandidate) {
-  paintTiming().markFirstPaint();
+  paintTiming().markFirstContentfulPaint();
   EXPECT_EQ(paintTiming().firstMeaningfulPaintCandidate(), 0.0);
   simulateLayoutAndPaint(1);
   double afterPaint = monotonicallyIncreasingTime();
@@ -105,4 +105,15 @@ TEST_F(FirstMeaningfulPaintDetectorTest, FirstMeaningfulPaintCandidate) {
   EXPECT_EQ(paintTiming().firstMeaningfulPaintCandidate(), candidate);
 }
 
+TEST_F(FirstMeaningfulPaintDetectorTest,
+       NetworkStableBeforeFirstContentfulPaint) {
+  paintTiming().markFirstPaint();
+  simulateLayoutAndPaint(1);
+  simulateNetworkStable();
+  EXPECT_EQ(paintTiming().firstMeaningfulPaint(), 0.0);
+  paintTiming().markFirstContentfulPaint();
+  simulateNetworkStable();
+  EXPECT_NE(paintTiming().firstMeaningfulPaint(), 0.0);
+}
+
 }  // namespace blink