DevTools: Get rid of synchronous XHRs in the frontend code

According to https://xhr.spec.whatwg.org/, sync XHRs on the main thread
have detrimental effects on the end user's experience and shall be removed.

R=vsevik

Review URL: https://codereview.chromium.org/685203003

git-svn-id: svn://svn.chromium.org/blink/trunk@185043 bbb929c8-8fbe-4397-9dbb-9b2b20218538