Fix an inconsistency in EnsureDefaultStyleSheetsForElement().

We don't need to go down into tag_name_ just to check for
MathML elements. This matches what we do for SVG.

Change-Id: Iabe2d7817267df581dcdee18e12bcc4137bd8762
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6243405
Auto-Submit: Steinar H Gunderson <sesse@chromium.org>
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1418089}