Media Controls: Add the feature flag to chrome://flags

Add the UseModernMediaControls feature flag to chrome://flags.

BUG=761305

Change-Id: I2f6bf9abf05d1256b9dab5fd444d1a8c7ccbbcd8
Reviewed-on: https://chromium-review.googlesource.com/698064
Reviewed-by: Mounir Lamouri <[email protected]>
Commit-Queue: Becca Hughes <[email protected]>
Cr-Commit-Position: refs/heads/master@{#513756}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 72d885e..4e9fb4e9 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -3617,9 +3617,14 @@
 #if defined(OS_CHROMEOS)
     {"native-samba", flag_descriptions::kNativeSambaName,
      flag_descriptions::kNativeSambaDescription, kOsCrOS,
-     FEATURE_VALUE_TYPE(features::kNativeSamba)}
+     FEATURE_VALUE_TYPE(features::kNativeSamba)},
 #endif  // defined(OS_CHROMEOS)
 
+    {"enable-modern-media-controls",
+     flag_descriptions::kUseModernMediaControlsName,
+     flag_descriptions::kUseModernMediaControlsDescription, kOsAll,
+     FEATURE_VALUE_TYPE(media::kUseModernMediaControls)}
+
     // NOTE: Adding a new flag requires adding a corresponding entry to enum
     // "LoginCustomFlags" in tools/metrics/histograms/enums.xml. See "Flag
     // Histograms" in tools/metrics/histograms/README.md (run the