Mark GPU.AtExit* metrics obsolete.

It doesn't look like these metrics have been recorded since M54.

Bug: none
Change-Id: I57aadf19add26f8d0d8d3d85e25f6fd9d599cbeb
Reviewed-on: https://chromium-review.googlesource.com/c/1313552Reviewed-by: Victor Miura <vmiura@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604990}

tools/metrics/histograms/histograms.xml

@@ -35766,6 +35766,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="GPU.AtExitMBytesAllocated" units="MB">
+  <obsolete>
+    Removed sometime around M55.
+  </obsolete>
   <owner>vmiura@chromium.org</owner>
   <summary>
     The amount of GPU memory that was currently allocated as of just before the
@@ -35774,6 +35777,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="GPU.AtExitMBytesAllocatedMax" units="MB">
+  <obsolete>
+    Removed sometime around M55.
+  </obsolete>
   <owner>vmiura@chromium.org</owner>
   <summary>
     The maximum amount of GPU memory that had ever been allocated during the GPU
@@ -35782,6 +35788,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="GPU.AtExitMBytesLimit" units="MB">
+  <obsolete>
+    Removed sometime around M55.
+  </obsolete>
   <owner>vmiura@chromium.org</owner>
   <summary>
     The GPU memory manager's limit on the amount of GPU memory that can be
@@ -35799,6 +35808,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="GPU.AtExitWindowCount">
+  <obsolete>
+    Removed sometime around M55.
+  </obsolete>
   <owner>vmiura@chromium.org</owner>
   <summary>
     The number of windows that the GPU memory manager believed to exist as of