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This copies what ExtensionRequestProtocolHandler does for the network service path, which is to load extension resources from the internet transparently if the manifest specifies it.

Bug: b/123321526
Change-Id: Iad8a5c525164907ef024fc1bf41825885f26fdba
Reviewed-on: https://chromium-review.googlesource.com/c/1481741
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635654}