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.cc b/chrome/browser/flag_descriptions.cc
index e2d5f76..a3af1bb 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -1457,6 +1457,10 @@
 const char kUseDdljsonApiDescription[] =
     "Enables the new ddljson API to fetch Doodles for the NTP.";
 
+const char kUseModernMediaControlsName[] = "New Media Controls";
+const char kUseModernMediaControlsDescription[] =
+    "Enables the new style native media controls.";
+
 const char kUserActivationV2Name[] = "User Activation v2";
 const char kUserActivationV2Description[] =
     "Enable simple user activation for APIs that are otherwise controlled by "