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This refactor will allow some manifest tests running in Chrome's unit_tests
suite to move to the extensions_unittests suite.

* Rename ExtensionManifestTest to extensions::ManifestTest and move it into
  src/extensions.
* Introduce ChromeManifestTest to load manifests from Chrome's test data dir.
* Eliminate some unnecessary use of chrome::VersionInfo::Channel.
* Move SharedModuleManifestTest to extensions_unittests as an example.

FileHandlerManifestTest and ExternallyConnectableManifestTest will be next,
but require more refactoring (e.g. of permissions).

BUG=397165
TEST=unit_tests, extensions_unittests

Review URL: https://codereview.chromium.org/572813002

Cr-Commit-Position: refs/heads/master@{#295087}