Make SliderThumbElement::shadowPseudoId and SliderContainerElement::shadowPseudoId robust.

Internals::shadowPseudoId can access shadowPseudoId any time. We can't assume a
renderer is attached to a shadow element.

This fixes test-only crashes, and makes Clusterfuzz happier.

BUG=345190
TEST=none

Review URL: https://codereview.chromium.org/181183002

git-svn-id: svn://svn.chromium.org/blink/trunk@167894 bbb929c8-8fbe-4397-9dbb-9b2b20218538