Remark svg/animations/additive-type-by-animation.html as leaky

This was accidentally removed in
https://chromium-review.googlesource.com/c/chromium/src/+/1060693/, which fixed
another set of leaks.

Bug: 878032
Change-Id: I70c6cf10bfb1f3d7e1552a1f637878a8322c7a39
Reviewed-on: https://chromium-review.googlesource.com/1191672Reviewed-by: Filip Gorski <fgorski@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586414}

third_party/WebKit/LayoutTests/LeakExpectations

@@ -74,6 +74,9 @@ crbug.com/874442 [ Linux ] svg/animations/mpath-remove-from-dependents-on-delete
 crbug.com/847114 [ Linux ] http/tests/devtools/tracing/decode-resize.js [ Pass Failure ]
 crbug.com/847114 [ Linux ] virtual/threaded/http/tests/devtools/tracing/decode-resize.js [ Pass Failure ]
 
+# Sheriff 2018-08-27
+crbug.com/878032 [ Linux ] svg/animations/additive-type-by-animation.html [ Pass Leak ]
+
 ###########################################################################
 # WARNING: Memory leaks must be fixed asap. Sheriff is expected to revert #
 # culprit CLs instead of suppressing the leaks. If you have any question, #