Media Router: Remove unnecessary and incorrect static_cast in test.

The SigninManager object here is in fact of type SigninManager, not
FakeSigninManagerForTesting. The bad cast here caused test failures
in CFI mode [1]. Because the test is not using any members of
FakeSigninManagerForTesting, we can simply remove the cast.

[1] https://build.chromium.org/p/chromium.fyi/builders/CFI%20Linux/builds/5924/steps/unit_tests/logs/MediaRouterContextualMenuUnitTest.Basic

BUG=464797
R=pkasting@chromium.org

Review-Url: https://codereview.chromium.org/2118503004
Cr-Commit-Position: refs/heads/master@{#403483}
1 file changed