Add metric with time between background tab being foregrounded and the first paint

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 PageLoadExtraInfo;
 struct PageLoadTiming;
 
-// Returns false for events for which we have no timing information, and events
-// that happened on a page that had been in the background. When a page is
-// backgrounded, some events (e.g. paint) are delayed. 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 PageLoadExtraInfo& info);
+// Returns true if:
+// - We have timing information for the event.
+// - The page load started while the page was in the foreground.
+// - The event occurred prior to the page being moved to the background.
+// When a page is backgrounded, some events (e.g. paint) are delayed. Since
+// these data points can skew the mean, they should not be mixed with timing
+// events that occurred in the foreground.
+bool StartInForegroundEventInForeground(base::TimeDelta event,

[kinuko, 2016/02/23 13:10:35]

I understand what these methods mean after reading this comment, but these still
feel a bit hard to parse-- maybe partly because this could be read as a function
that starts something in foreground at a first look.

Actually, I wonder how bad it is if we just define
EventOccuredInForeground(event, info) helper and do write

if (info.started_in_foreground && EventOccuredInForeground(event, info))

if (!info.started_in_foreground && EventOccuredInForeground(event, info))

..?

[pkotwicz, 2016/02/23 19:04:52]

StartInForegroundEventInForeground() represents an important concept during
metrics recording so I want to keep it as one function.

I have renamed the function to WasStartedInForegroundEventInForeground() to make
it clear that the function does not start anything.

+                                        const PageLoadExtraInfo& info);
+
+
+// Returns true if:
+// - We have timing information for the event.
+// - The page load started while the page was in the background.
+// - The event occurred after the page was moved to the foreground and prior to
+//   the page being moved back to the background.
+bool StartInBackgroundEventInForeground(base::TimeDelta event,
+                                        const PageLoadExtraInfo& info);
 
 }  // namespace page_load_metrics
 
 #endif  // COMPONENTS_PAGE_LOAD_METRICS_BROWSER_PAGE_LOAD_METRICS_UTIL_H_