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On case-insensitive filesystems, extensions can generate requests
(script src tags, XHR's, etc.) to their own resources using a
relative path with incorrect case and have those requests
work (see crbug.com/29941 for some history). However, for
extension content verification, we were looking up the expected
file content hashes using the relative path given in the request,
not the actual filename, which meant that any difference in case
would be treated as "no hashes for this file".

This patch switches to using case-insensitive lookups, but uses a
multimap so that case-sensitive filesystems should not experience
problems.

BUG=412693

TEST=Install the test extension at http://goo.gl/rOpGDu, and turn
on content verification to Enforce mode in about:flags. Without
this patch, the extension will get force disabled on
windows/mac. With the patch, this should be fixed.

Review URL: https://codereview.chromium.org/585583003

Cr-Commit-Position: refs/heads/master@{#297032}