Don't fail the style assert in Element::rendererIsFocusable if the document is active.

This avoids crashing in call sites that happen during document setup and shutdown
of the document. An example of the former appears to be in the attached bug.

An example of the latter is easily reproducible: load a document with hyperlinks
in it, select some text via the text finding widget (ctrl-f), then click on the 
hyperlink. The code to close the find box will call into Blink and check
if the elements are focusable. However, the elements no longer have renderers
since the document has been deactivated to prepare for navigation.

TBR=abarth@chromium.org
BUG=251163

Review URL: https://codereview.chromium.org/533963002

git-svn-id: svn://svn.chromium.org/blink/trunk@181497 bbb929c8-8fbe-4397-9dbb-9b2b20218538