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This CL changes timing of stacking notification counter to appear in
UnifiedSystemTray. Combined with https://crrev.com/c/1215507, it fixes
top corner artifact of ARC++ notifications in UnifiedSystemTray.

TEST=manual
BUG=866777

Change-Id: I57f0ca846b129c18e7500d7738375644ff82157b
Reviewed-on: https://chromium-review.googlesource.com/1215602
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590177}