Revert of Support margin-top for legend in fieldset. (patchset #5 id:80001 of https://codereview.chromium.org/2396813002/ )

Reason for revert:
need to revert the previously landed patch http://crrev.com/415577 that blocks the release

Original issue's description:
> Support margin-top for legend in fieldset.
>
> The implementation aligns with Edge, Gecko centers the legend+margins, which
> looks undesirable to me.  The specs say nothing about this.
>
> The new behaviour causes two tests to change, both have been rebaselined. One
> was additionally modified to not trigger the scrollbar, which in turn would
> render differently on various platforms.
>
> BUG=554077
> CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
>
> Committed: https://crrev.com/462e2f8e1a0f0a09efa7c8bcc34da36fd7b532ca
> Cr-Commit-Position: refs/heads/master@{#425127}

TBR=eae@chromium.org,karlo@opera.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=554077
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2558683002
Cr-Commit-Position: refs/heads/master@{#436891}
14 files changed