[metrics] Mark long-removed metrics as obsolete.

Affected metrics:
 . Compositing.Browser.PicturePile.UpdateInvalidatedAreaPerMs
 . Compositing.Browser.PicturePile.UpdateUs
 . Compositing.Browser.RasterTask.RasterUs
 . Compositing.Renderer.PicturePile.UpdateInvalidatedAreaPerMs
 . Compositing.Renderer.PicturePile.UpdateUs
 . Compositing.Renderer.RasterTask.RasterUs

BUG=1088690, 1088688, 1089795

Change-Id: Ie8d1dc15647b9876987a5566626eb5ab002d09f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285478
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786233}

tools/metrics/histograms/histograms.xml

@@ -27774,6 +27774,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Compositing.Browser.PicturePile.UpdateInvalidatedAreaPerMs"
     units="pixels/ms" expires_after="M85">
+  <obsolete>
+    Obsolete since 2015.
+  </obsolete>
   <owner>paint-dev@chromium.org</owner>
   <summary>
     Area of invalidated content, in pixels, divided by update (i.e. record), in
@@ -27783,6 +27786,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Compositing.Browser.PicturePile.UpdateUs" units="microseconds"
     expires_after="M85">
+  <obsolete>
+    Obsolete since 2015.
+  </obsolete>
   <owner>paint-dev@chromium.org</owner>
   <summary>
     Time spent updating (i.e. recording) a picture pile, in microseconds.
@@ -27825,6 +27831,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Compositing.Browser.RasterTask.RasterUs" units="microseconds"
     expires_after="M85">
+  <obsolete>
+    Removed 04/2019. Was unused.
+  </obsolete>
   <owner>paint-dev@chromium.org</owner>
   <summary>
     Time spent completing a compositor rasterization task, in microseconds.
@@ -28650,6 +28659,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Compositing.Renderer.PicturePile.UpdateInvalidatedAreaPerMs"
     units="pixels/ms" expires_after="M85">
+  <obsolete>
+    Obsolete since 2015.
+  </obsolete>
   <owner>paint-dev@chromium.org</owner>
   <summary>
     Area of invalidated content, in pixels, divided by update (i.e. record), in
@@ -28659,6 +28671,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Compositing.Renderer.PicturePile.UpdateUs"
     units="microseconds" expires_after="M85">
+  <obsolete>
+    Obsolete since 2015.
+  </obsolete>
   <owner>paint-dev@chromium.org</owner>
   <summary>
     Time spent updating (i.e. recording) a picture pile, in microseconds.
@@ -28701,6 +28716,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Compositing.Renderer.RasterTask.RasterUs" units="microseconds"
     expires_after="M85">
+  <obsolete>
+    Removed 04/2019. Was unused.
+  </obsolete>
   <owner>paint-dev@chromium.org</owner>
   <summary>
     Time spent completing a compositor rasterization task, in microseconds.