remove use of {,un}setPendingActivity from DOMFileSystem.

{,un}setPendingActivity is deprecated. This CL replaces it with |hasPendingActivity()| override.

This also makes FileSystemCallbacks reference to the DOMFileSystem strong, as |addPendingCallbacks()| would no longer ref() DOMFileSystem.

TEST=fast/filesystem
BUG=354845

Review URL: https://codereview.chromium.org/212263002

git-svn-id: svn://svn.chromium.org/blink/trunk@170052 bbb929c8-8fbe-4397-9dbb-9b2b20218538