Make SVG elements not to consider editable.

This patch changes Position::isCandidate() and Position::isCandidate() not to
return true for SVG elements except for rendered text, since we don't implement
SVG editor yet.

Note: Rendered text in SVG documents are represented by |Text| class and their
renderer is |RenderSVGInlineText| derived from |RenderText| class.

BUG=380043
TEST=LayoutTests/editing/execCommand/format-block-with-uneditable-crash-2.html

Review URL: https://codereview.chromium.org/314923003

git-svn-id: svn://svn.chromium.org/blink/trunk@175855 bbb929c8-8fbe-4397-9dbb-9b2b20218538