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This is a reland of f01abd36

The original patch failed on Windows bots because there was no
"/tmp" directory. This new patch saves the generated layers-overlay.html
in the results directory (existence ensured).

Original change's description:
> Update paint/invalidation/repaint-overlay/layers-overlay.html
>
> It is generated by a python test case
> (TestRepaintOverlay.test_generate_repaint_overlay_html).
> I should have updated it when changing repaint_overlay.py.
>
> Updated the test to fail and print a message if the file needs an
> update.
>
> Bug: 789272
> Change-Id: I2e6ba6573788177dfbc09d2bd4d3d80023d4b21f
> Reviewed-on: https://chromium-review.googlesource.com/794875
> Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
> Reviewed-by: Philip Rogers <pdr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#519912}

TBR=pdr@chromium.org

Bug: 789272
Change-Id: I17de00b123520be148bba4aacd923f21d3f18e9c
Reviewed-on: https://chromium-review.googlesource.com/797110Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520171}