Tell people that enums.txt doesn't need OWNER review.

This is placed in big letters at the top of the file to have a chance
of being noticed by someone when their editor opens before moving down
to find the entry they want to change.

Change-Id: I204d14138dc508a640ced8c17008c8ae95c1bdb7
Reviewed-on: https://chromium-review.googlesource.com/1015787Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551736}

tools/metrics/histograms/enums.xml

@@ -5,6 +5,20 @@ found in the LICENSE file.
 -->
 
 <!--
+
+        You
+
+                   888                  d8b 888
+                   888                  88P 888
+                   888                  8P  888
+               .d88888  .d88b.  88888b. "   888888
+              d88" 888 d88""88b 888 "88b    888
+              888  888 888  888 888  888    888
+              Y88b 888 Y88..88P 888  888    Y88b.
+               "Y88888  "Y88P"  888  888     "Y888
+
+        need an OWNER review to submit chages to this file.
+
 This file describes the enumerations used in metrics data.  It is used
 along with histograms.xml to describe the histogram data collected by Chrome.