The test as it currently is, is broken. The test originally used to set the command-line flag, and then create a ScopedViewsTestHelper, which would cause mus to launch through PlatformTestHelperMus, with the new cmd-flag set by the test (--force-device-scale-factor=2). But this changed in https://crrev.com/393912, where mus would start before any tests, and then would live for the rest of the tests. This means mus never sees the flag set by the test, causing the expectation failure.

The test does work when run in isolation.