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If file browser handlers are specified in the manifest, and the permission is
missing, then they will be ignored. This CL adds a warning so users are aware
of it.

TEST=unit_tests: *FileBrowserHandlerManifestTest.InvalidFileBrowserHandlers*
BUG=470494

Review URL: https://codereview.chromium.org/1017163003

Cr-Commit-Position: refs/heads/master@{#322530}