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

components/page_load_metrics/browser/metrics_web_contents_observer.cc

Index: components/page_load_metrics/browser/metrics_web_contents_observer.cc
diff --git a/components/page_load_metrics/browser/metrics_web_contents_observer.cc b/components/page_load_metrics/browser/metrics_web_contents_observer.cc
index 81747999c028915585e5ee3799c708978daacb5d..21130fa48e099d66c5a0a96865dcc28282b0fb88 100644
--- a/components/page_load_metrics/browser/metrics_web_contents_observer.cc
+++ b/components/page_load_metrics/browser/metrics_web_contents_observer.cc
@@ -159,14 +159,13 @@ void PageLoadTracker::LogAbortChainHistograms(
 
 void PageLoadTracker::WebContentsHidden() {
   // Only log the first time we background in a given page load.
-  if (started_in_foreground_ && background_time_.is_null())
+  if (background_time_.is_null())
     background_time_ = base::TimeTicks::Now();

[Bryan McQuade, 2016/02/25 16:10:51]

similar to below, can you add

// Make sure we either started in the foreground and haven't been foregrounded
yet, or started in the background and have already been foregrounded.
DCHECK_EQ(started_in_foreground_, foreground_time_.is_null());

 }
 
 void PageLoadTracker::WebContentsShown() {
   // Only log the first time we foreground in a given page load.
-  // Don't log foreground time if we started foregrounded.
-  if (!started_in_foreground_ && foreground_time_.is_null())
+  if (foreground_time_.is_null())
     foreground_time_ = base::TimeTicks::Now();

[Bryan McQuade, 2016/02/25 16:10:51]

To make sure WebContentsShown and WebContentsHidden are called in the way we
expect, I'd like to sanity check that the first time we get a WebContentsShown,
we either started in the background and haven't been backgrounded yet, or
started in the foreground and have already been backgrounded. To do this, can
you add
// Make sure we either started in the background and haven't been backgrounded
yet, or started in the foreground and have already been backgrounded.
DCHECK_NE(started_in_foreground_, background_time_.is_null());

 }
 
@@ -227,9 +226,9 @@ PageLoadExtraInfo PageLoadTracker::GetPageLoadMetricsInfo() {
   base::TimeDelta first_foreground_time;
   base::TimeDelta time_to_abort;
   base::TimeDelta time_to_commit;
-  if (!background_time_.is_null() && started_in_foreground_)
+  if (!background_time_.is_null())
     first_background_time = background_time_ - navigation_start_;
-  if (!foreground_time_.is_null() && !started_in_foreground_)
+  if (!foreground_time_.is_null())
     first_foreground_time = foreground_time_ - navigation_start_;
   if (abort_type_ != ABORT_NONE) {
     DCHECK_GT(abort_time_, navigation_start_);