Make MediaSession a runtime-enabled feature on Desktop.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index c29e053cdd14d1dbd2cb8e25f875dbbac5a7e616..f138f0be71758c052374199f526e6225e089b659 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -497,8 +497,10 @@ test("content_browsertests") {
             content_tests_gypi_values.content_browsertests_android_sources,
             ".",
             "//content")
-    sources -=
-        [ "../browser/battery_status/battery_monitor_impl_browsertest.cc" ]
+    sources -= [
+      "../browser/battery_status/battery_monitor_impl_browsertest.cc",
+      "../browser/media/session/media_session_delegate_default_browsertest.cc",
+    ]
     deps += [
       ":content_browsertests_java",
       ":content_browsertests_manifest",