clang/win: Fix build after https://codereview.chromium.org/1248543002/

Fixes:
..\..\media\audio\win\audio_device_listener_win_unittest.cc(24,19) : error:
unused variable 'kNoDevice' [-Werror,-Wunused-const-variable]
static const char kNoDevice[] = "";
                  ^
No intended behavior change.

BUG=506712,82385
TBR=dalecurtis@chromium.org

Review URL: https://codereview.chromium.org/1250933002.

Cr-Commit-Position: refs/heads/master@{#339820}

media/audio/win/audio_device_listener_win_unittest.cc

@@ -21,7 +21,6 @@ using base::win::ScopedCOMInitializer;
 
 namespace media {
 
-static const char kNoDevice[] = "";
 static const char kFirstTestDevice[] = "test_device_0";
 static const char kSecondTestDevice[] = "test_device_1";