Unmark document-policy-oversized-images-resize.php as flaky

The CL which caused the flakiness was reverted in
(https://chromium-review.googlesource.com/c/chromium/src/+/2218833).

The cause was attaching request params to image requests causing
multiple requests being sent(previously only one request with one url).
Upon first layout, different number of requests complete causes
flaky behaviour.

Unmark the test as flaky.

Bug: 1087208
Change-Id: I5124c9107b50500962e82e63c6f846f557d6f282
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261255Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782029}

third_party/blink/web_tests/TestExpectations

@@ -6760,7 +6760,6 @@ crbug.com/1083585 [ Win ] geolocation-api/watchPosition-page-visibility.html [ P
 crbug.com/1046784 http/tests/devtools/elements/styles/stylesheet-tracking.js [ Pass Failure Timeout ]
 
 # Sheriff 2020-05-28
-crbug.com/1087208 http/tests/images/document-policy-oversized-images-resize.php [ Pass Failure ]
 crbug.com/1083585 [ Linux ] geolocation-api/watchPosition-page-visibility.html [ Pass Failure ]
 crbug.com/1087077 external/wpt/html/semantics/forms/form-submission-0/form-double-submit.html [ Failure ]
 crbug.com/1087077 external/wpt/html/semantics/forms/form-submission-0/form-double-submit-3.html [ Failure ]