Add download button to MediaDocument on Android.

content/public/common/content_features.cc

Index: content/public/common/content_features.cc
diff --git a/content/public/common/content_features.cc b/content/public/common/content_features.cc
index e6c1c61cad0a89348d30c16f118ad56c258fb3f8..eff329731f87c28bc53527563b47110d91355bd4 100644
--- a/content/public/common/content_features.cc
+++ b/content/public/common/content_features.cc
@@ -28,6 +28,13 @@ const base::Feature kDownloadResumption{"DownloadResumption",
 const base::Feature kExperimentalFramework{"ExperimentalFramework",
                                            base::FEATURE_ENABLED_BY_DEFAULT};
 
+// FeatureList definition for trials to enable the download button on
+// MediaDocument.
+const base::Feature kMediaDocumentDownloadButton{
+    "MediaDocumentDownloadButton",
+    base::FEATURE_DISABLED_BY_DEFAULT
+};
+
 // Non-validating reload on reload-to-refresh-content (e.g. pull-to-refresh).
 // See https://crbug.com/558829
 const base::Feature kNonValidatingReloadOnRefreshContent{