Don't log background navigations in StaleWhileRevalidateExperiment histograms

components/page_load_metrics/browser/page_load_metrics_util.h

 // 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.
 
 #ifndef COMPONENTS_PAGE_LOAD_METRICS_BROWSER_PAGE_LOAD_METRICS_UTIL_H_
 #define COMPONENTS_PAGE_LOAD_METRICS_BROWSER_PAGE_LOAD_METRICS_UTIL_H_
 
 #include "base/metrics/histogram_macros.h"
 #include "base/time/time.h"
 
 #define PAGE_LOAD_HISTOGRAM(name, sample)                           \
   UMA_HISTOGRAM_CUSTOM_TIMES(name, sample,                          \
                              base::TimeDelta::FromMilliseconds(10), \
                              base::TimeDelta::FromMinutes(10), 100)
 
 namespace page_load_metrics {
 
 struct PageLoadTiming;
 
 // Get the time of the first 'contentful' paint. A contentful paint is a paint
 // that includes content of some kind (for example, text or image content).
 // Painting of a background color is not considered 'contentful'.
 base::TimeDelta GetFirstContentfulPaint(const PageLoadTiming& timing);
 
+// Returns false for events for which we have no timing information, and events
+// that happened after the page was backgrounded. When a page is backgrounded,
+// the priority is lowered, and the load timing events no longer reflect the

[Charlie Harrison, 2015/12/08 20:26:21]

I don't know if the priority is always lowered. rdsmith@ might know more. I
would also add that some events (e.g. paint) are delayed until foreground, which
is the root cause of many long tailed histograms.

[Adam Rice, 2015/12/09 06:29:00]

I reworded the comment. PTAL.

+// user experience. Since these data points can skew the mean, they should not
+// be mixed with timing events that occurred in the foreground.
+bool EventOccurredInForeground(const base::TimeDelta& event,
+                               const base::TimeDelta& first_background);
+
 }  // namespace page_load_metrics
 
 #endif  // COMPONENTS_PAGE_LOAD_METRICS_BROWSER_PAGE_LOAD_METRICS_UTIL_H_