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This patch implements computing positions of overlines and
'text-underline-position: under' underlines.

Computing consistent positions across boxes within a same decoration
box is not implemented in this patch.

In doing so, the code to computing font vertical position is moved to
SimpleFontData.

Bug: 714962
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_layout_ng;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I27b471db76ae0c94b7cc01ce2eb9084fa06ff29b
Reviewed-on: https://chromium-review.googlesource.com/886283
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532118}