SVG: Do not allow nested <textPath>

We have rejected <textPath><textPath></textPath></textPath> in layout
trees. However we failed to reject <textPath><a><textPath>...

NGSvgTextLayoutAttributesBuilder has an assumption that <textPath> can
not be nested, and had a crash.

Bug: 1246627
Change-Id: Ib8ac869ae5301e26a614d5b87ff724e5057458bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3140801


Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918495}