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This matches the old path's mime_sniffing_resource_handler.cc.

Bug: 729849
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I7afd5ac3d595ddcea5b061dd064e4413eab590df
Reviewed-on: https://chromium-review.googlesource.com/1173764Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582916}