Split out Ash dependency in app info dialog unittests

Split out Ash dependency in app info dialog unittests

The app info dialog doesn't have an ash dependency on Mac, so this needs
to be isolated.

The non-Ash tests have both an Extensions and a Views dependency. The
extension system requires a BrowserThreadBundle (from content), which
was previously provided by AshTestBase. However, ViewsTestBase does not
(and shouldn't) have a content dependency. Instead, it can use the
BrowserThreadBundle provided by TestExtensionEnvironment which has some
other useful things.

Then, to satisfy the Views dependency, we still can't use ViewsTestBase
because it has its own MessageLoop that will fight with the
BrowserThreadBundle. A follow-up will introduce a ScopedViewsTestHelper
to allow the test to run on Mac, but for now just use AshTestBase.

In the AppInfo unittests, make better use of TestExtensionEnvironment to
reuse common code.

BUG=485854, 412234

Review URL: https://codereview.chromium.org/1175783003

Cr-Commit-Position: refs/heads/master@{#334533}