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Removes the assumption that InstallableManager currently has: that it will
continue to exist after calling the callback. This makes sense if considering
the callback as a return-like statement for the async GetData call.

In prep for refactoring in http://crrev.com/c/1873817

Bug: 1007860
Change-Id: I18f751b32c88a9d3cc9d6c3f3a083fd3952ebe9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884409
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710242}