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Loading an extension is just like loading a platform app (but without
sending a launch event). So we can support --load-extension in
AppShell.

Also call extension_system->Init() unconditionally.

NOPRESUBMIT=true # existing ScopedAllowIO test usage is allowed

Bug: 742646
Change-Id: Id3c2adbebddc62d22953399f98e781229bda6d2b
Reviewed-on: https://chromium-review.googlesource.com/570809
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487394}