cros: Fix TranslateManagerRenderViewHostTest crashes in official builds
The EulaAcceptedNotifier is trying to read a preference on Chrome OS official
builds, but unit_tests don't have a PrefService (and don't need one). Skip the
pref lookup when we don't have a PrefService, which is what other tests do.
With this change all unit_tests pass on Chrome OS official/branded builds.
BUG=430637
TEST=unit_tests
Review URL: https://codereview.chromium.org/
706753002
Cr-Commit-Position: refs/heads/master@{#302920}