Check for a null element in Internals::elementLayerTreeAsText

This patch is a followup to [1] which failed to check for a null
element when calling the internals-only elementLayerTreeAsText api.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=169579

TEST=Manual, call internals.elementLayerTreeAsText with no element.
BUG=356596

Review URL: https://codereview.chromium.org/218393002

git-svn-id: svn://svn.chromium.org/blink/trunk@170420 bbb929c8-8fbe-4397-9dbb-9b2b20218538