ash: Disable PeripheralBatteryNotifierTest.StylusNotification.

Flaky on --mus.

TBR=oshima@chromium.org

Test: none
Bug: 765794
Change-Id: I61cae8ce1b1e016ad1dfb2dd294b043ef548e409
Reviewed-on: https://chromium-review.googlesource.com/831342Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524579}

ash/system/power/peripheral_battery_notifier_unittest.cc

@@ -183,7 +183,8 @@ TEST_F(PeripheralBatteryNotifierTest, DISABLED_DeviceRemove) {
               nullptr);
 }
 
-TEST_F(PeripheralBatteryNotifierTest, StylusNotification) {
+// TODO(crbug.com/765794): Flaky on ash_unittests --mus.
+TEST_F(PeripheralBatteryNotifierTest, DISABLED_StylusNotification) {
   const std::string kTestStylusBatteryPath =
       "/sys/class/power_supply/hid-AAAA:BBBB:CCCC.DDDD-battery";
   const std::string kTestStylusName = "test_stylus";