Disable MediaNotificationServiceCastTest.CountCastSessionsAsActive on Linux and Win

Test is flaky on Linux and Windows:
https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyWQsSBUZsYWtlIk5jaHJvbWl1bUB1bml0X3Rlc3RzQE1lZGlhTm90aWZpY2F0aW9uU2VydmljZUNhc3RUZXN0LkNvdW50Q2FzdFNlc3Npb25zQXNBY3RpdmUM

TBR=takumif@chromium.org

Bug: 1034406
Change-Id: I5403d8163546553597490123dce4aec167601cd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028265Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736509}

chrome/browser/ui/global_media_controls/media_notification_service_unittest.cc

@@ -647,8 +647,8 @@ TEST_F(MediaNotificationServiceTest, DismissesMediaSession) {
       1);
 }
 
-// TODO(https://crbug.com/1034406) Flaky on Mac10.12
-#if defined(OS_MACOSX)
+// TODO(https://crbug.com/1034406) Flaky on Mac10.12, Linux and Win10.
+#if defined(OS_MACOSX) || defined(OS_LINUX) || defined(OS_WIN)
 #define MAYBE_CountCastSessionsAsActive DISABLED_CountCastSessionsAsActive
 #else
 #define MAYBE_CountCastSessionsAsActive CountCastSessionsAsActive