SVG: Fix handling of lang and xml:lang attributes

Specification: https://svgwg.org/svg2-draft/struct.html#LangSpaceAttrs

SVGElement should handle lang and xml:lang attributes in the same way
as HTMLElement.

This CL affects <text> rendering and dynamic :lang() selector
matching.
Note that SVG elements has supported :lang() matching, but dynamic
update of lang/xml:lang didn't invalidate selector matching correctly.

* element.{h,cc} and html_element.{h,cc}:
  Move MapLanguageAttributeToLocale() from HTMLElement to Element.

* runtime_enabled_features.json5:
  Add a runtime flag just in case, and enable it.

Bug: 1370460
Change-Id: Ifdfd7d16d3c9560a6eacc371e2c198a5e0e3a383
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3932377
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1054500}