LeakExpectations: fix the duplicated entry callstack-placards-discarded.html
LeakExpectations: fix the duplicated entry callstack-placards-discarded.html

The leak detector bots is red because of the duplicated entry (http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Leak/builds/1279).

NOTRY=true
BUG=376188, 364408, 371264
R=kouhei@chromium.org

Review URL: https://codereview.chromium.org/296903008

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@174672 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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