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}
1 file changed