DevTools: [Documentation] Check article existence when adding context menu items

We should check if the requested property is documented
(against the list of articles from webplatform.org)
before creating a context menu item for it.

Drive-by fix: loadXHR method from utilities.js was remade for using it with Promises.

BUG=391593, 407097

Review URL: https://codereview.chromium.org/479173002

git-svn-id: svn://svn.chromium.org/blink/trunk@180958 bbb929c8-8fbe-4397-9dbb-9b2b20218538