Have Document::mediaQueryMatcher() / Document::selectorQueryCache() return references

Have Document::mediaQueryMatcher() / Document::selectorQueryCache() return references
instead of pointers as they can never return null. This makes the calling code look
safer.

R=adamk
BUG=

Review URL: https://codereview.chromium.org/33013004

git-svn-id: svn://svn.chromium.org/blink/trunk@160357 bbb929c8-8fbe-4397-9dbb-9b2b20218538