Use new downcast helper for blink::SVGForeignObjectElement

This CL has two goals,
  1. Use To<SVGForeignObjectElement> as new downcast helper
  2. Use IsA<SVGForeignObjectElement>(element) in place of
     IsSVGForeignObjectElement(element)

Bug: 891908
Change-Id: I6ad9fe6116758ba05ea8e7339b71405fc1f9348b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936585
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720813}
8 files changed