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Before this change, PostAsyncResults returned a value that was not
explicitly initialized if the IAsyncOperation failed. This was not
causing a problem in the code today because ComPtrs are implicitly
initialized to a valid (though empty) state and all of the consumers of
this code currently are for IAsyncOperations that return types that
inherit from IUnknown (which causes the PostAsyncResults method to
return them inside a ComPtr). If, however, PostAsyncResults is called
for an IAsyncOperation that returns a different kind of value (e.g. an
int), the returned value was unitialized memory (in the failure case).

To protect against future use of PostAsyncResults with non-IUnknown
types (like that planned as part of http://crbug.com/1035527) this
change adds default value initialization of the result, as well as unit
tests that validate this behavior and a test-helper class for
simulating IAsyncOperations.

Bug: 1121285
Change-Id: If87d817c3cff8830a9e91eeb8fa51d2c32d17f01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2373165Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Hoch Hochkeppel <mhochk@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#802241}