Log an immediate variant of FirstContentfulPaint

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 0d4089fe06121b4fd455451ccc42457d2055f136..001f02141f2b9c19703dcf8aa101ab0fb7cea41b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35542,9 +35542,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="PageLoad.Timing2.NavigationToFirstContentfulPaint" units="ms">
   <owner>ksakamoto@chromium.org</owner>
   <summary>
-    The time from navigation start to first &quot;contentful&quot; paint. For
-    now, this is the minimum of PageLoad.Timing2.NavigationToFirstTextPaint and
-    PageLoad.Timing2.NavigationToFirstImagePaint.
+    The time from navigation start to first &quot;contentful&quot; paint.

[Bryan McQuade, 2016/05/06 18:27:03]

note: the latter part of the description was no longer accurate, so i removed
it.

   </summary>
 </histogram>
 
@@ -90912,6 +90910,16 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="InputMethod.VirtualKeyboard.InitLatency"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="ImmediatePageLoadTiming" separator=".">
+  <suffix name="Immediate"
+      label="Logged immediately after a timing update is observed, unlike
+             normal PageLoad timing metrics, which are logged at the end of a
+             page's lifetime. This metric is temporary and is intended to
+             help loading-dev to understand the impact of logging metrics at
+             the end of a page load."/>
+  <affected-histogram name="PageLoad.Timing2.NavigationToFirstContentfulPaint"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="IndexedDBLevelDBErrnoMethods" separator=".">
   <suffix name="NewLogger" label="ChromiumEnv::NewLogger"/>
   <suffix name="NewSequentialFile" label="ChromiumEnv::NewSequentialFile"/>