LeakExpectations: Add callstack-placards-discarded.html and some tests which time out

BUG=371264
NOTRY=true

Review URL: https://codereview.chromium.org/278583002

git-svn-id: svn://svn.chromium.org/blink/trunk@173610 bbb929c8-8fbe-4397-9dbb-9b2b20218538