Set Ash material design mode in tests properly

Currently, in Ash tests, material design mode is set after call to
AshTestBase::SetUp(). This is not right as some setup in AshTestBase
might depend on material mode. This CL allows setting material mode
before calling AshTestBase::SetUp().

BUG=620093

Review-Url: https://codereview.chromium.org/2186363002
Cr-Commit-Position: refs/heads/master@{#408837}
10 files changed