Make MediaQueryMatcher::styleResolverChanged() safe after disconnected from Document

This patch changes MediaQueryMatcher::styleResolverChanged() to do nothing when |m_document| is null instead of assertion. This situation can be happened in FrameLoader::clear(), which disassociates MediaQueryMatcher and Document, set m_document to null, then calls FrameSelection::prepareFoDescturction(), which calls Document::recalcStyle().

There is another patch r158088 which checks null Frame, although it isn't enough for fixing issue 294829.

BUG=294829
TEST=n/a; hard to re-produce

Review URL: https://codereview.chromium.org/48273010

git-svn-id: svn://svn.chromium.org/blink/trunk@160821 bbb929c8-8fbe-4397-9dbb-9b2b20218538