chrome/blink: Add functionality for in-product help for media elements.

third_party/WebKit/Source/core/frame/Settings.json5

 {
   // Defines properties which are available on the Settings object.
   //
   // Please think carefully before adding a new Setting.  Some questions to
   // consider are:
   // - Should this be a RuntimeEnabledFeature instead? Settings are for things
   //   which we support either values of at runtime.  Features are set at renderer
   //   process startup and are never changed. Features also tend to be set to a
   //   value based on the platform or the stability of the code in question, where
   //   as settings both codepaths need to be stable.
@@ skipping 935 matching lines @@
     },
     {
       name: "highContrastContrast",
       initial: 0,
       type: "double",
     },
     {
       name: "highContrastImagePolicy",
       initial: "HighContrastImagePolicy::kFilterAll",
       type: "HighContrastImagePolicy",
+    },
+    {
+      name: "mediaDownloadInProductHelpEnabled",
+      initial: false,
     }
   ],
 }