DevTools: Get rid of supportsFileSystems method in InspectorFrontendHost.

BUG=337487
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/149523002

git-svn-id: svn://svn.chromium.org/blink/trunk@166105 bbb929c8-8fbe-4397-9dbb-9b2b20218538