Decided to only tack it on to Startup.FirstWebContents.MainNavigationStart and as such to not bother about NTP since it shouldn't matter.

Startup.FirstWebContents.MainNavigationStart.SingleTab should then be our ground truth for noise-free startups.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/19f104a391020bf6538f3fc78ce3c022096796ae

commit 19f104a391020bf6538f3fc78ce3c022096796ae
Author: gab <gab@chromium.org>
Date: Mon Feb 06 19:29:41 2017

Add .SingleTab and .MultiTabs suffix to Startup.FirstWebContents.MainNavigationStart histogram.

BUG= 684046 

Review-Url: https://codereview.chromium.org/2651823002
Cr-Commit-Position: refs/heads/master@{#448348}

[modify] https://crrev.com/19f104a391020bf6538f3fc78ce3c022096796ae/chrome/browser/metrics/OWNERS
[modify] https://crrev.com/19f104a391020bf6538f3fc78ce3c022096796ae/chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc
[modify] https://crrev.com/19f104a391020bf6538f3fc78ce3c022096796ae/chrome/browser/metrics/first_web_contents_profiler.cc
[modify] https://crrev.com/19f104a391020bf6538f3fc78ce3c022096796ae/chrome/browser/metrics/first_web_contents_profiler.h
[modify] https://crrev.com/19f104a391020bf6538f3fc78ce3c022096796ae/components/startup_metric_utils/browser/startup_metric_utils.cc
[modify] https://crrev.com/19f104a391020bf6538f3fc78ce3c022096796ae/components/startup_metric_utils/browser/startup_metric_utils.h
[modify] https://crrev.com/19f104a391020bf6538f3fc78ce3c022096796ae/tools/metrics/histograms/histograms.xml

+CC occasional startup performance trackers FYI.

Startup.FirstWebContents.MainNavigationStart.SingleTab((.Cold|LukeWarm|Warm)Startup) is now on M58 Beta. The difference between .SingleTab and .Multitabs isn't as large as I'd expected but .SingleTab will still be a better metric for "browser startup" that further reduces the noise.

https://uma.googleplex.com/p/chrome/histograms/?endDate=20170319&dayCount=3&histograms=Startup.FirstWebContents.MainNavigationStart.MultiTabs.ColdStartup%2CStartup.FirstWebContents.MainNavigationStart.MultiTabs.WarmStartup%2CStartup.FirstWebContents.MainNavigationStart.SingleTab.ColdStartup%2CStartup.FirstWebContents.MainNavigationStart.SingleTab.WarmStartup&fixupData=true&showMax=true&filters=channel%2Ceq%2C3%2Cplatform%2Ceq%2CW%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial