[Extensions] Update external extension warning to allow new platforms
Certain platforms (currently only Windows) prompt for externally-
installed, or sideloaded, extensions. In the future, we may want to
spread this to other platforms, but we'll want to grandfather in any
extensions that were already installed.
Update the external installation disablement and warning code to only
affect new installation of extensions. Add additional unit tests for:
- The general case of installing and disabling an external extension (I
couldn't find any for this already in place, though some probably test
it transitively?).
- Extensions installed before the feature is turned on not being
disabled.
- Not warning about extensions installed before the feature is turned
on.
BUG=669277
Review-Url: https://codereview.chromium.org/2607673002
Cr-Commit-Position: refs/heads/master@{#441000}
4 files changed
