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Currently, ObjectProxy::CallMethod is implemented by creating ErrorCallback from ResponseCallback.
This does not work with OnceCallback because it's not allowed to create two callbacks from one OnceCallback.
To solve this, this change introduces CallMethodCallbackInternal to ObjectProxy which takes either Response* or ErrorResponse*, and dispatches the response to the appropriate callback.

Also, Gmock does not allow directly mocking functions which take move-only
arguments.
https://github.com/google/googletest/blob/master/googlemock/docs/CookBook.md#mocking-methods-that-use-move-only-types
To workaround this, add DoCallMethod to MockObjetProxy.

BUG=740015
TEST=build

Change-Id: I1a98b5fd1d6d70f69674102a3d66bcec0c13214c
Reviewed-on: https://chromium-review.googlesource.com/578541
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488615}