chromeos: removes AshTestEnvironment

AshTestEnvironment was doing two things:
. responsible for creating the appropriate ScopedTaskEnvironment.
. creating a ViewsDelegate.

All subclasses created the same ViewsDelegate, so that could be removed.
ScopedTaskEnvironment subclasses are only needed for Chrome code, so
add an explicit destroy of existing ScopedTaskEnvironment and have chrome
create its own. This means ash can get rid of test_support_with_content
and test_support_without_content.

BUG=none
TEST=none
[email protected]

Change-Id: I916554619b276ca0ebd16aeb3b0fde51a73b61b0
Reviewed-on: https://chromium-review.googlesource.com/c/1383041
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Xiyuan Xia <[email protected]>
Commit-Queue: Scott Violet <[email protected]>
Cr-Commit-Position: refs/heads/master@{#617863}
38 files changed