Delay filesystem API async callbacks while JS debugger is on pause.

While the Javascript VM is paused all callbacks into the javascipt from C++ bindings are ignored.
For async filesystem API callbacks we can postpone the callbacks while debugger is on pause, rather than just ignore them.

BUG=370100
R=kinuko@chromium.org, nhiroki@chromium.org

Review URL: https://codereview.chromium.org/303523002

git-svn-id: svn://svn.chromium.org/blink/trunk@174865 bbb929c8-8fbe-4397-9dbb-9b2b20218538