This moves the stroke approximation code from
LayoutSVGPath::HitTestStrokeBoundingBox to a helper in LayoutSVGShape,
and then makes use of it in LayoutSVGShape::CalculateStrokeBoundingBox()
(which is the generic "stroke bounds" computation for shapes) as well.

This makes stroke bounding box computation O(1) (assuming a fill bbox
exists), avoiding the generation of a stroke path, which can be
expensive for the more "complex" parametrizations (primarily dashes.)

The downside is of course lower fidelity, and the approximation will
always be both larger than the fill bbox and larger than the "exact"
stroke bbox.

Also fix an issue where a miter-limit < 1 would incorrectly shrink the
resulting bounding box in some cases.

Bug: 435097
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Id0228bc78bb7d8db223be082106f3c3d2bcb038a
Reviewed-on: https://chromium-review.googlesource.com/774746
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518392}