Make ApplyStyleCommand::applyBlockStyle to work with non-HTML element.

This patch changes assumption to check a block to apply style as |HTMLElement|
to |Element| in |ApplyStyleCommand::applyBlockStyle()| to work with non-HTML
element, a namespaceURI of element is other than HTML namespace URI.

Note: We can create and insert non-HTML element by document.createElementNS().

BUG=387750
TEST=LayoutTests/editing/execCommand/remove-format-with-non-html-element-crash.html

Review URL: https://codereview.chromium.org/356443003

git-svn-id: svn://svn.chromium.org/blink/trunk@177283 bbb929c8-8fbe-4397-9dbb-9b2b20218538