Pass blocking pool runner to CreateTemporaryFile in SyncFS v2 drive_backend

MessageLoopProxy::current() could be null otherwise, which hits assertion

BUG=240165
R=tzik@chromium.org

Review URL: https://codereview.chromium.org/98063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238021 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238021