Android: Session.TotalDuration metric may not be accurate if a dialog is displayed.

Details from Bryan McQuade:

"
On Android, we record Session.TotalDuration during this flow, which is described as "The length of a session (launch/foregrounding to backgrounding) in milliseconds."

Unfortunately, it seems that onPause can be called in cases where the app isn't really going into the background. In particular, in https://developer.android.com/guide/topics/processes/process-lifecycle.html, when describing different process/activity states, it says:

"A visible process ... is running an Activity that is visible to the user on-screen but not in the foreground (its onPause() method has been called). This may occur, for example, if the foreground Activity is displayed as a dialog that allows the previous Activity to be seen behind it."

I've confirmed that if Chrome displays a dialog, due to for example a JavaScript alert, onPause gets invoked, and we log Session.TotalDuration at this point, even though Chrome is in the foreground. When the dialog gets dismissed, we begin recording a new session. This means both that time spent displaying dialogs is not included in session duration (not necessarily a bad thing), but also that we end up recording a single Chrome session as multiple separate sessions, which means both our session counts and individual session duration tracking logic are incorrect (though the sum total of all durations is correct, excluding time spent with dialogs displayed).

In my case, we saw a somewhat significant number of cases where the onPause->OnAppEnterBackground flow was invoked without Chrome going into the background: as a result of updating the page load metrics 'hidden' state as part of the OnAppEnterBackground path and updating our visibility, the reported number of page loads that went into the background more than doubled. So it seems that this happens with enough frequency that it's probably worth trying to address it."

One solution that was proposed was to delay recording the session end for a few seconds, which could allow merging sessions that would be interrupted by a dialog - but a concern was brought up that this could cause us to lose sessions when the app gets killed after going to the background.

For now, I'll land a CL to put this disclaimer in the histogram description.