Reenable test sandboxed-iframe-plugins.html

[Note to sheriff]
Feel free to immediately revert this CL if this test is failing
in the next 3 days.

This test was marked flaky 2 years ago, but nobody was assigned or CC.
As a result, it hasn't been properly investigated. This patch enables it again.
If it works, great. If it doesn't work we will get useful data about it.

R=mkwst@chromium.org

Bug: 745887
Change-Id: I0c50d61992dd56a13aee891e4eedfd14c43bc9df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108144
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751620}

third_party/blink/web_tests/TestExpectations

@@ -4816,9 +4816,6 @@ crbug.com/1052642 virtual/cache-storage-high-priority-match/external/wpt/service
 # Sheriff failures 2017-07-03
 crbug.com/708994 http/tests/security/cross-frame-mouse-source-capabilities.html [ Timeout Pass ]
 
-crbug.com/745887 [ Mac ] fast/frames/sandboxed-iframe-plugins.html [ Failure Pass ]
-crbug.com/745887 [ Win ] fast/frames/sandboxed-iframe-plugins.html [ Failure Pass ]
-
 crbug.com/746128 [ Win7 Debug ] media/controls/video-enter-exit-fullscreen-without-hovering-doesnt-show-controls.html [ Failure ]
 crbug.com/746128 [ Win7 Debug ] virtual/audio-service/media/controls/video-enter-exit-fullscreen-without-hovering-doesnt-show-controls.html [ Failure ]
 crbug.com/746128 [ Mac ] media/controls/video-enter-exit-fullscreen-without-hovering-doesnt-show-controls.html [ Failure Pass ]