Handle has_more correctly in WebFileSystemImpl

Make WaitableCallbackResults ref-counted and a member of WebFileSystemImpl to keep it alive longer than before.
Add CallbacksUnregisterMode argument to RunCallbacks() to keep callbacks registered when necessary.
Initialize next_callbacks_id_ with 1 instead 0 to make 0 usable as an invalid ID.
Implement waitForAdditionalResult().
UnregisterCallbacks() is also responsible to unregister WaitableCallbackResults.

Corresponding patch: https://codereview.chromium.org/178333009/

BUG=347900
TEST=run_webkit_tests.sh http/tests/filesystem/\*
TEST=run_webkit_tests.sh http/tests/inspector/filesystem/\*

Review URL: https://codereview.chromium.org/188533002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255625 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255625