Add a note to OilpanLeakExpectations for future gardeners

TBR=haraken@chromium.org
NOPRESUBMIT=true
NOTRY=true
BUG=581987

Review URL: https://codereview.chromium.org/1642883002

Cr-Commit-Position: refs/heads/master@{#372062}

third_party/WebKit/LayoutTests/OilpanLeakExpectations

@@ -4,5 +4,7 @@
 # This list isn't comprehensive yet, and Leak Detector bots are not tree closers atm.
 # Gardeners are welcome to touch this file, but you can ignore the failures
 # and wait for kouhei@ or hajimehoshi@ to maintain this file.
-
-crbug.com/581987 harness-tests/mojo-helpers.html [ Leak Failure ]
+#
+# Also, while Oilpan is enabled on ToT it will not read leak expectations from this
+# file. So gardeners, put Oilpan leak expectations in LeakExpectations in a section
+# labelled "Oilpan" or some such.