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

TBR=jchaffraix@chromium.org
BUG=380151, 378716
NOTRY=true

Review URL: https://codereview.chromium.org/312693004

git-svn-id: svn://svn.chromium.org/blink/trunk@175364 bbb929c8-8fbe-4397-9dbb-9b2b20218538