This fixes an issue where you saved a change but then deleted it.
But then you decided to edit the same file again, this resulted in
it restoring from the cache.
Another example is editing a file then restoring it to it's previous state.
Because it was cached it would restore it from the cache which is not
was supposed to be happening.
The cause of this was eraseEditableContentItem missing a call to
_getEditableContentKey.
Bug: Issue 10371
Change-Id: I47e4ca216804f0a86b1cec4ac549bb0599e17cb2
621d8b3457