While investigating bug 1243549, we encountered several instances of the following error message during each startup:

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A coding exception was thrown and uncaught in a Task.

Full message: TypeError: this.Paths is null
Full stack: Agent.wipe@resource:///modules/sessionstore/SessionWorker.js:296:7
worker.dispatch@resource:///modules/sessionstore/SessionWorker.js:21:24
anonymous/AbstractWorker.prototype.handleMessage@resource://gre/modules/workers/PromiseWorker.js:122:16
@resource:///modules/sessionstore/SessionWorker.js:30:41

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These messages can be explained as follows:

* If sanitization has failed during shutdown, it attempts again to
  sanitize during startup. This happens more often than it used to,
  because of 1/ startup bug fixes in bug 1089695; 2/ new shutdown bugs
  most likely also added by or around bug 1089695.

* Sanitization during startup doesn't wait until Session Restore has
  properly started to sanitize the session. So sanitization of Session
  Restore file fails. This has probably always been the case, except
  we never noticed.

* For some reason I do not understand, it attempts to sanitize several
  times.

* I suspect that this can cause problems during startup, as
  sanitization and Session Restore race to use/remove the files of
  Session Restore.

This patch makes sure that SessionFile.wipe() waits until
initialization of SessionFile is complete before proceeding.

--HG--
extra : source : a768b93e7f0f5e63d2b907d0e09d7c08247f9942
extra : intermediate-source : 932d8e4fcf3aa927c3fac8b4535d67e5b4638138