GMC: Only enable by default for Windows, Mac, and Linux

This CL modifies the logic for enabling GMC by default. This fixes an
issue where GMC was enabled by default on ChromeOS.

Bug: 995836
Change-Id: I6b38423043a9c50480dc255eb9a12dc2500cb714
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761779Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688694}

media/base/media_switches.cc

@@ -269,8 +269,15 @@ 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
+};
 
 // Enable new cpu load estimator. Intended for evaluation in local
 // testing and origin-trial.