Refactoring: Page::console should return a reference.

It is never null.  Also, an argument of ScriptPreprocessor constructor should
be PageConsole& because only Page::console is passed to it.

This change prevents one from adding unnecessary NULL check in the future.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/22889023

git-svn-id: svn://svn.chromium.org/blink/trunk@156527 bbb929c8-8fbe-4397-9dbb-9b2b20218538