Refactoring: Page::editorClient and Editor::client should return references.

Page::editorClient is never null.
Editor::client can't access a Page in some cases, and we use
EmptyEditorClient in such cases.

BUG=
TEST=none; This should not make behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23112008

git-svn-id: svn://svn.chromium.org/blink/trunk@156280 bbb929c8-8fbe-4397-9dbb-9b2b20218538