Revert "Mark only-valid-drop-targets-receive-file-drop.html as being flaky."

This reverts r175364. The test has been fixed and is confirmed to
not flake anymore.

BUG=380151
NOTRY=true

Review URL: https://codereview.chromium.org/353003003

git-svn-id: svn://svn.chromium.org/blink/trunk@177081 bbb929c8-8fbe-4397-9dbb-9b2b20218538