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

components/page_load_metrics/browser/metrics_web_contents_observer.cc

 // 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.
 
 #include "components/page_load_metrics/browser/metrics_web_contents_observer.h"
 
 #include <utility>
 
 #include "base/location.h"
 #include "base/logging.h"
@@ skipping 141 matching lines @@
                            aborted_chain_size_);
       return;
     default:
       NOTREACHED();
       return;
   }
 }
 
 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());

 }
 
 void PageLoadTracker::Commit(content::NavigationHandle* navigation_handle) {
   // TODO(bmcquade): To improve accuracy, consider adding commit time to
   // NavigationHandle. Taking a timestamp here should be close enough for now.
   commit_time_ = base::TimeTicks::Now();
   committed_url_ = navigation_handle->GetURL();
   for (const auto& observer : observers_) {
     observer->OnCommit(navigation_handle);
   }
@@ skipping 39 matching lines @@
 void PageLoadTracker::AddObserver(
     scoped_ptr<PageLoadMetricsObserver> observer) {
   observers_.push_back(std::move(observer));
 }
 
 PageLoadExtraInfo PageLoadTracker::GetPageLoadMetricsInfo() {
   base::TimeDelta first_background_time;
   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_);
     time_to_abort = abort_time_ - navigation_start_;
   } else {
     DCHECK(abort_time_.is_null());
   }
   if (!committed_url_.is_empty()) {
     DCHECK_GT(commit_time_, navigation_start_);
     time_to_commit = commit_time_ - navigation_start_;
@@ skipping 308 matching lines @@
 
   if (!committed_load_->UpdateTiming(timing)) {
     // If the page load tracker cannot update its timing, something is wrong
     // with the IPC (it's from another load, or it's invalid in some other way).
     // We expect this to be a rare occurrence.
     RecordInternalError(ERR_BAD_TIMING_IPC);
   }
 }
 
 }  // namespace page_load_metrics