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Previously ChromeOS used same implementation with linux which returns mime type by reading mimetype-extension database in the OS.
However it doesn't worked since ChromeOS doesn't have it.
This CL implements ChromeOS specific logic which returns mime type based on hard-coded list.

BUG=472017, 477913, 488378
TEST=net_unittests:MimeUtilTest.ExtensionTest

Review URL: https://codereview.chromium.org/1134393002

Cr-Commit-Position: refs/heads/master@{#330482}