GitLab

Share the logic with MimeSniffingResourceHandler.

Bug: 778721
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I4a90beee86a5182b0a8d1c6225c4aaa711abed7d
Reviewed-on: https://chromium-review.googlesource.com/745661
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512776}