Add MetricsProvider::OnAppEnterBackground callback.

components/metrics/metrics_service.cc

Index: components/metrics/metrics_service.cc
diff --git a/components/metrics/metrics_service.cc b/components/metrics/metrics_service.cc
index cad15984141323b821d374be362e103287d88dc8..5a3269fc3c823b07e587e50ec20a14a776cad053 100644
--- a/components/metrics/metrics_service.cc
+++ b/components/metrics/metrics_service.cc
@@ -474,6 +474,11 @@ void MetricsService::OnAppEnterBackground() {
   // persisting all logs. Unlinke a shutdown, the state is primed to be ready
   // to continue logging and uploading if the process does return.
   if (recording_active() && state_ >= SENDING_LOGS) {
+    // Give providers a chance to persist histograms as part of being
+    // backgrounded.
+    for (MetricsProvider* provider : metrics_providers_)

[Bryan McQuade, 2016/07/27 13:18:34]

I wasnt't sure if we should be informing providers here, inside the test to see
if recording is active, or before this if test, even if recording is not active.
My sense it it's only useful to implement this callback if histograms are also
going to be persisted, and my understanding is they'll only be persisted if we
enter the 'if' case above, so it makes sense to invoke the callback here. But I
don't understand the code as well, so that may not be the case. Please take a
look and let me know which you think is correct.

[Alexei Svitkine (slow), 2016/07/27 17:41:11]

It's debatable, but I somewhat prefer doing it outside the if. For example, if
the state later changes, it would be good to have those metrics have been
logged.

[Bryan McQuade, 2016/07/27 18:25:15]

Good point, I agree. I moved it.

+      provider->OnAppEnterBackground();
+
     PushPendingLogsToPersistentStorage();
     // Persisting logs closes the current log, so start recording a new log
     // immediately to capture any background work that might be done before the