DevTools: Extract RemoteObject.getProperty method.

So that it could be reused soon.

BUG=
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/34983003

git-svn-id: svn://svn.chromium.org/blink/trunk@160487 bbb929c8-8fbe-4397-9dbb-9b2b20218538