Add missing null check in PageSerializer::serializeCSSStyleSheet()

Add missing null check in PageSerializer::serializeCSSStyleSheet() for
importRule->styleSheet(). CSSImportRule::styleSheet() can return null but
the function it is being passed to expects a non-null pointer.

Also update PageSerializer::serializeCSSStyleSheet() to take its CSSStyleSheet
argument by reference, instead of pointer to make it obvious to the caller
that passing null is not OK.

R=morrita@chromium.org

Review URL: https://codereview.chromium.org/332573003

git-svn-id: svn://svn.chromium.org/blink/trunk@176123 bbb929c8-8fbe-4397-9dbb-9b2b20218538