Revert "Add tentative WPT to verify download in sandbox"

This reverts commit 726375b6a0c5583f4d6c37a0d5edaf2421cb72b9.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 626289 as the
culprit for flakes in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/flake/flake-culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyQwsSDEZsYWtlQ3VscHJpdCIxY2hyb21pdW0vNzI2Mzc1YjZhMGM1NTgzZjRkNmMzN2EwZDVlZGFmMjQyMWNiNzJiOQw

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.mac/Mac10.13%20Tests/9272

Sample Failed Step: webkit_layout_tests on Intel GPU on Mac

Sample Flaky Test: external/wpt/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_navigation_download_allow_downloads_without_user_activation.sub.tentative.html

Original change's description:
> Add tentative WPT to verify download in sandbox
> 
> General testing idea:
> For a network request, we store a token in stash. And after a fixed
> period of time sufficient for the download to finish, if there are
> any, we validate the token in the stash to see if the network request
> was issued and finished successfully.
> 
> In the case of <a download> where the decision of download can be made
> before resource fetching, the server sets the token immediately.
> 
> In the case of navigation to a download, the server will stream a
> response over 1 seconds and set the token at the end only when the
> socket has been connected. So it is able to detect any download
> cancellation while fetching the resource.
> 
> Bug: 539938
> Change-Id: I734c8cc18d1f761f16646c6b859a6b731ab40ff5
> Reviewed-on: https://chromium-review.googlesource.com/c/1422667
> Commit-Queue: Yao Xiao <yaoxia@chromium.org>
> Reviewed-by: Mike West <mkwst@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#626289}

Change-Id: Id343b8de2a751856b27907ba4c8dfb52715e4598
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 539938
Reviewed-on: https://chromium-review.googlesource.com/c/1437999
Cr-Commit-Position: refs/heads/master@{#626323}
7 files changed