De-flake only-valid-drop-targets-receive-file-drop.

Looks like after Blink r175269 the moment where the test runner
grabs the text dump may occur after the quick-fail timer.
I've only been able to repro it once out of about 40 invocations
so I can only speculate that the navigation which we cancel in
beforeunload is somethimes delaying the test's completion long
enough for the deadman's switch to fire.

Since the order of events is correct even in the failing case
it's relatively simple to make the test pass regardless of whether
the test runner grabs the text dump before or after the timer fires.

BUG=380151

Review URL: https://codereview.chromium.org/319673002

git-svn-id: svn://svn.chromium.org/blink/trunk@175696 bbb929c8-8fbe-4397-9dbb-9b2b20218538