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Updates the resource manager to not require a mime type when loading
a bundled resource. This is the same as the behavior in [1].

[1] https://cs.chromium.org/chromium/src/extensions/browser/extension_protocols.cc?q=extension_protocols.cc&sq=package:chromium&dr&l=340

Bug: 902646
Change-Id: Ib65eafd6fc900351a9270317a711c9d3ab092f80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501194
Commit-Queue: dstockwell <dstockwell@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638324}