DevTools: refactor InspectorStyleSheet

Introduce InspectorStyleSheetBase, inherit InspectorStyleSheet and InspectorStyleSheetForInlineStyle from it.

BUG=341506

Review URL: https://codereview.chromium.org/202103002

git-svn-id: svn://svn.chromium.org/blink/trunk@169550 bbb929c8-8fbe-4397-9dbb-9b2b20218538