Add P_TO_Q upgrade type to enum.xml.

BUG=b:132979912
TEST=CQ is more than enough.

Change-Id: Idcb4be6661cfa7ea6f9eb2661499b3797f536eb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626152Reviewed-by: Satoshi Niwa <niwa@chromium.org>
Commit-Queue: Garfield Tan <xutan@chromium.org>
Auto-Submit: Garfield Tan <xutan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662803}

tools/metrics/histograms/enums.xml

@@ -2479,6 +2479,7 @@ Unknown properties are collapsed to zero. -->
   <int value="3" label="M_TO_N"/>
   <int value="4" label="M_TO_P"/>
   <int value="5" label="N_TO_P"/>
+  <int value="6" label="P_TO_Q"/>
 </enum>
 
 <enum name="ArcShareFilesOnExit">