Paper over a recent null-ptr crash regression in SVGImage

I don't like this patch, but this unblock's Karen's
ability to roll, while we decide if we're going to
roll out the recent SVGImage page-destruction deferal
code or if we can add a test for this ASSERT and
clarify the lifetime so that others who might expect
to still have a Page object don't have similar confusion.

BUG=350907
TBR=haraken

Review URL: https://codereview.chromium.org/193323003

git-svn-id: svn://svn.chromium.org/blink/trunk@168861 bbb929c8-8fbe-4397-9dbb-9b2b20218538