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This comprises style spans like <b> and <i>, as well as text links of
the form <link1>.

Also refactored testing util for style spans.

Bug: b/145043394
Change-Id: I7bcd375db919ad050a4edaace00f2cff1cc61085
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2116233Reviewed-by: Marian Fechete <marianfe@google.com>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#753566}