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As a preparation step to deprecate resource_messages.h, this change
migrates ResourceDispatcherHostTest to LoadingWithMojo. This change
also removes some tests that are no longer needed.

Bug: 788241
Change-Id: I04ca5649fd621404a177d9c2571384a00a0d7741
Reviewed-on: https://chromium-review.googlesource.com/818925
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523957}