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This patch implements Range.getClientRects and getBoundingClientRect
by implemeting LayoutText::AbsoluteQuadsForRange, the underlying
function to compute quads for a range of text.

VisibleUnits and TouchAdjustment are other users of this function.

crbug.com/698038 added some logic to clear the quad vector under
specific condition for legacy, but the logic is a bit complicated.
This patch tries to match to the spec without using the same logic.
There may be a need to tweak the logic a little more, but the new
logic seems to be more interoperable with Edge/Gecko.

Bug: 636993, 755750
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I5c6323680285a4e03d64109cd5681c788f6a6bbf
Reviewed-on: https://chromium-review.googlesource.com/979732Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546079}