Mark http/tests/images/png-partial-load-as-document.html flaky on Mac and Win 7

Example failures:
https://ci.chromium.org/buildbot/chromium.mac/Mac10.10%20Tests/29357
https://ci.chromium.org/buildbot/chromium.mac/Mac10.11%20Tests/23562
https://ci.chromium.org/buildbot/chromium.mac/Mac10.12%20Tests/10337
https://ci.chromium.org/buildbot/chromium.webkit/WebKit%20Win7/60528

TBR=hiroshige@chromium.org

Bug: 813704
Change-Id: I2632b5acd189b9a203700c8b5b1f57fc449aba93
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/925870Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537713}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3311,3 +3311,5 @@ crbug.com/812941 [ Win7 ] external/wpt/css/css-multicol/multicol-inherit-003.xht
 crbug.com/812941 [ Win7 ] external/wpt/css/css-multicol/multicol-rule-color-inherit-001.xht [ Failure Pass ]
 crbug.com/812941 [ Win7 ] external/wpt/css/css-multicol/multicol-span-all-margin-nested-firstchild-001.xht [ Failure Pass ]
 crbug.com/812941 [ Win7 ] external/wpt/css/css-multicol/multicol-table-cell-vertical-align-001.xht [ Failure Pass ]
+
+crbug.com/813704 [ Win7 Mac ] http/tests/images/png-partial-load-as-document.html [ Failure Pass ]