[CI] Add LayoutSVGBlock::GetElement helper

Like the equivalent helper on LayoutSVGModelObject, this provides a more
specific type of Node (an SVGElement), since is guaranteed for
subclasses of LayoutSVGBlock.
Also update the documentation in LayoutSVGModelObject to reflect the
current name.

Bug: 769774
Change-Id: I720af53e5a24e960773059f7d2a113e74bd5bdc5
Reviewed-on: https://chromium-review.googlesource.com/973378
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545034}
5 files changed