Retire fast/dom/drop-event-for-input-in-shadow.html leak expectation.

Following r184768, test no longer leaks.

TBR=kouhei
BUG=364417
NOTRY=true

Review URL: https://codereview.chromium.org/696183002

git-svn-id: svn://svn.chromium.org/blink/trunk@184781 bbb929c8-8fbe-4397-9dbb-9b2b20218538