Expand expiry of some PageLoad.Internal histograms

This CL updates the expiry of histograms that are bound to expire soon.
It also updates the descriptions since the semantics of the histograms
changed in place after commit a5484e63.

Bug: 1144816
Change-Id: I0eaf28a4f59c7d5803dcda9acd0f36e6678e8bab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527507Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826288}

tools/metrics/histograms/histograms_xml/page/histograms.xml

@@ -1565,30 +1565,29 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram
     name="PageLoad.Internal.PaintTiming.ExperimentalLargestContentfulPaint.ContentType"
-    enum="LargestContentType" expires_after="2020-12-11">
+    enum="LargestContentType" expires_after="2021-12-11">
   <owner>npm@chromium.org</owner>
   <owner>speed-metrics-dev@chromium.org</owner>
   <summary>
     Measures whether the largest contentful paint, whose timestamp is measured
-    by PageLoad.PaintTiming.NavigationToExperimentalLargestContentfulPaint,
-    comes from text or image. This value is recorded whenever
-    PageLoad.PaintTiming.NavigationToExperimentalLargestContentfulPaint is
-    recorded.
+    by PageLoad.PaintTiming.NavigationToLargestContentfulPaint, comes from text
+    or image. This value is recorded whenever
+    PageLoad.PaintTiming.NavigationToLargestContentfulPaint is recorded.
   </summary>
 </histogram>
 
 <histogram
     name="PageLoad.Internal.PaintTiming.ExperimentalLargestContentfulPaint.MainFrame.ContentType"
-    enum="LargestContentType" expires_after="2020-12-11">
+    enum="LargestContentType" expires_after="2021-12-11">
   <owner>npm@chromium.org</owner>
   <owner>speed-metrics-dev@chromium.org</owner>
   <summary>
-    Measures whether the largest content paint in the main frame, whose
+    Measures whether the largest contentful paint in the main frame, whose
     timestamp is measured by
-    PageLoad.PaintTiming.NavigationToExperimentalLargestContentfulPaint.MainFrame,
-    comes from text or image. This value is recorded whenever
-    PageLoad.PaintTiming.NavigationToExperimentalLargestContentfulPaint.MainFrame
-    is recorded.
+    PageLoad.PaintTiming.NavigationToLargestContentfulPaint.MainFrame, comes
+    from text or image. This value is recorded whenever
+    PageLoad.PaintTiming.NavigationToLargestContentfulPaint.MainFrame is
+    recorded.
   </summary>
 </histogram>
 
@@ -1600,9 +1599,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   <owner>speed-metrics-dev@chromium.org</owner>
   <summary>
     Measures whether the largest contentful paint, whose timestamp is measured
-    by PageLoad.PaintTiming.NavigationToLargestContentfulPaint, comes from text
+    by PageLoad.PaintTiming.NavigationToLargestContentfulPaint2, comes from text
     or image. This value is recorded whenever
-    PageLoad.PaintTiming.NavigationToLargestContentfulPaint is recorded.
+    PageLoad.PaintTiming.NavigationToLargestContentfulPaint2 is recorded.
   </summary>
 </histogram>
 
@@ -1613,11 +1612,11 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   <owner>npm@chromium.org</owner>
   <owner>speed-metrics-dev@chromium.org</owner>
   <summary>
-    Measures whether the largest content paint in the main frame, whose
+    Measures whether the largest contentful paint in the main frame, whose
     timestamp is measured by
-    PageLoad.PaintTiming.NavigationToLargestContentPaint.MainFrame, comes from
-    text or image. This value is recorded whenever
-    PageLoad.PaintTiming.NavigationToLargestContentfulPaint.MainFrame is
+    PageLoad.PaintTiming.NavigationToLargestContentfulPaint2.MainFrame, comes
+    from text or image. This value is recorded whenever
+    PageLoad.PaintTiming.NavigationToLargestContentfulPaint2.MainFrame is
     recorded.
   </summary>
 </histogram>