CSS highlight painting: fix incorrect TextDecorationInfo storage

Since highlights can break text into many parts that are painted in
overlay order, and within each part the decorations are ordered by
type *then* by overlay, computing TextDecorationInfo on the fly can
be very wasteful.

CL:3608432 computes the TextDecorationInfo once per layer, caching it
in Vector<LayerPaintState> along with each layer’s ComputedStyle and
TextPaintStyle, but TextDecorationInfo has been STACK_ALLOCATED since
at least CL:2637834, so storing it in a Vector like this is unsound.

TextDecorationInfo could be downgraded to DISALLOW_NEW, but for now,
this patch moves the TextDecorationInfo cache to its own stack array
(CachedDecorationInfo[2]), which should be enough for most content.
If the text fragment has more than two layers with decorations, any
excess layers will compute TextDecorationInfo on the fly.

Fixed: 1404921
Change-Id: I200837ab0408b8310f815c62e003af4132b686c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4183741
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Delan Azabani <dazabani@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1095361}