Allow the externalfile scheme to be whitelisted as an allowed scheme for component extensions
The "externalfile" scheme is used by ChromeOS to provide local access to Drive
files. Component extensions can have access to URLs at these schemes as long as <all_urls>
permission is specified. However currently, even if they have access, this scheme
isn't added as an allowed scheme in Blink. This adds the scheme if it is permitted.
BUG=453202
Review URL: https://codereview.chromium.org/
885263002
Cr-Commit-Position: refs/heads/master@{#316040}