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Some existing tests do not fully verify the API usage. This CL is
dedicated to provide a better coverage for OpenXR implementation.
And remove some no longer used mock functions.

Fixed: 1025404
Change-Id: I7493d5ddc6f00544d8f8474fdaa3ab2fe8d283c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919494
Commit-Queue: Zheng Qin <zheqi@microsoft.com>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718386}