[TestExpectations] Unmark png-partial-load-as-document.html

The test seems no longer flaky.
A couple of flakes on dashboard seem unrelated.
(e.g. "with patch" runs with many other tests failing)

Test: http/tests/images/png-partial-load-as-document.html
Bug: 813704
Change-Id: Idc9629bb4cf9b2f80af15f96d5f1d97938548b7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2120997Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753573}

third_party/blink/web_tests/TestExpectations

@@ -5078,8 +5078,6 @@ crbug.com/794338 virtual/audio-service/media/video-rotation.html [ Failure Pass
 crbug.com/811605 media/video-poster-after-loadedmetadata.html [ Failure Pass ]
 crbug.com/811605 virtual/audio-service/media/video-poster-after-loadedmetadata.html [ Failure Pass ]
 
-crbug.com/813704 http/tests/images/png-partial-load-as-document.html [ Failure Pass ]
-
 crbug.com/806645 [ Win7 ] http/tests/devtools/elements/elements-panel-rewrite-href.js [ Failure Pass ]
 crbug.com/806645 [ Mac ] http/tests/devtools/elements/elements-panel-rewrite-href.js [ Failure Pass ]