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/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index ac1d55e..8dfb052 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -895,6 +895,9 @@
 extern const char kUseDdljsonApiName[];
 extern const char kUseDdljsonApiDescription[];
 
+extern const char kUseModernMediaControlsName[];
+extern const char kUseModernMediaControlsDescription[];
+
 extern const char kUserActivationV2Name[];
 extern const char kUserActivationV2Description[];