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This CL converts MediaSourceTest to use GetMimeTypeForFile() to get the
mime type for test files to remove duplicate code.

Bug: 897767
Test: No functionality change
Change-Id: I5953cf8ae4e466d6fbb9c5d0ea021077a6f27f7f
Reviewed-on: https://chromium-review.googlesource.com/c/1296998Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602131}