Export FirstMeaningfulPaintCandidate() through WebPerformance

FMP signal has sometimes been used for dismissing a splash screen in some
downstream Web Application platforms. But, FMP signal couldn't occasionally
be generated or happened a little bit late, then it could cause that the
splash screen dismissed a little bit late in the Web Application platforms.
This CL is to export FirstMeaningfulPaintCandidate() in order to be used when
FirstMeaningfulPaint() doesn't work or generates a signal a little bit late.

Bug: 848639
Test: Covered by FirstMeaningfulPaintDetectorTest.FirstMeaningfulPaintCandidate
Change-Id: I492d98ef4998e772d293f81b9991a066cb346233
Reviewed-on: https://chromium-review.googlesource.com/1080499Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung.kim@lge.com>
Cr-Commit-Position: refs/heads/master@{#564841}

third_party/blink/public/web/web_performance.h

@@ -90,6 +90,7 @@ class WebPerformance {
   BLINK_EXPORT double FirstImagePaint() const;
   BLINK_EXPORT double FirstContentfulPaint() const;
   BLINK_EXPORT double FirstMeaningfulPaint() const;
+  BLINK_EXPORT double FirstMeaningfulPaintCandidate() const;
   BLINK_EXPORT double PageInteractive() const;
   BLINK_EXPORT double PageInteractiveDetection() const;
   BLINK_EXPORT double FirstInputInvalidatingInteractive() const;

third_party/blink/renderer/core/exported/web_performance.cc

@@ -166,6 +166,11 @@ double WebPerformance::FirstMeaningfulPaint() const {
   return MillisecondsToSeconds(private_->timing()->FirstMeaningfulPaint());
 }
 
+double WebPerformance::FirstMeaningfulPaintCandidate() const {
+  return MillisecondsToSeconds(
+      private_->timing()->FirstMeaningfulPaintCandidate());
+}
+
 double WebPerformance::PageInteractive() const {
   return MillisecondsToSeconds(private_->timing()->PageInteractive());
 }

third_party/blink/renderer/core/timing/performance_timing.cc

@@ -349,6 +349,15 @@ unsigned long long PerformanceTiming::FirstMeaningfulPaint() const {
   return MonotonicTimeToIntegerMilliseconds(timing->FirstMeaningfulPaint());
 }
 
+unsigned long long PerformanceTiming::FirstMeaningfulPaintCandidate() const {
+  const PaintTiming* timing = GetPaintTiming();
+  if (!timing)
+    return 0;
+
+  return MonotonicTimeToIntegerMilliseconds(
+      timing->FirstMeaningfulPaintCandidate());
+}
+
 unsigned long long PerformanceTiming::PageInteractive() const {
   InteractiveDetector* interactive_detector = GetInteractiveDetector();
   if (!interactive_detector)

third_party/blink/renderer/core/timing/performance_timing.h

@@ -100,6 +100,13 @@ class CORE_EXPORT PerformanceTiming final : public ScriptWrappable,
   // The time of the first 'meaningful' paint, A meaningful paint is a paint
   // where the page's primary content is visible.
   unsigned long long FirstMeaningfulPaint() const;
+  // The time of the candidate of first 'meaningful' paint, A meaningful paint
+  // candidate indicates the first time we considered a paint to qualify as the
+  // potential first meaningful paint. But, be careful that it may be an
+  // optimistic (i.e., too early) estimate.
+  // TODO(crbug.com/848639): This function is exposed as an experiment, and if
+  // not useful, this function can be removed.
+  unsigned long long FirstMeaningfulPaintCandidate() const;
   // The first time the page is considered 'interactive'. This is determined
   // using heuristics based on main thread and network activity.
   unsigned long long PageInteractive() const;