DevTools: Replace CSS.getStyleSheet with CSS.getStyleSheetRules in devtools protocol.

This patch also removes get/setText methods from WebInspector.CSSStyleSheet object. This object should be eventually merged with WebInspector.CSSStyleSheetHeader.

Review URL: https://codereview.chromium.org/116433004

git-svn-id: svn://svn.chromium.org/blink/trunk@164082 bbb929c8-8fbe-4397-9dbb-9b2b20218538