GMC: Disable for ChromeOS

When I enabled GMC by default, I accidentally enabled it for ChromeOS
(again, see crrev.com/c/1761779). This CL makes GMC only enabled for
Windows, Mac, and Linux.

Bug: 1045550
Change-Id: I6ae57cae0569c05a03da0c1f15763d97dc1b5f85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023045Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735873}

media/base/media_switches.cc

@@ -326,8 +326,15 @@ const base::Feature kFallbackAfterDecodeError{"FallbackAfterDecodeError",
                                               base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Show toolbar button that opens dialog for controlling media sessions.
-const base::Feature kGlobalMediaControls{"GlobalMediaControls",
-                                         base::FEATURE_ENABLED_BY_DEFAULT};
+const base::Feature kGlobalMediaControls {
+  "GlobalMediaControls",
+#if defined(OS_WIN) || defined(OS_MACOSX) || \
+    (defined(OS_LINUX) && !defined(OS_CHROMEOS))
+      base::FEATURE_ENABLED_BY_DEFAULT
+#else
+      base::FEATURE_DISABLED_BY_DEFAULT
+#endif
+};
 
 // Auto-dismiss global media controls.
 const base::Feature kGlobalMediaControlsAutoDismiss{