Add a note that user gesture is required to display the media galleries dialog.

chrome/common/extensions/api/media_galleries.idl

Index: chrome/common/extensions/api/media_galleries.idl
diff --git a/chrome/common/extensions/api/media_galleries.idl b/chrome/common/extensions/api/media_galleries.idl
index f1a92a4894b6ee2dd11563ee7e82a5e5a165f802..d2267bae8f98f908d285fc663e4a53e34b9e0dd2 100644
--- a/chrome/common/extensions/api/media_galleries.idl
+++ b/chrome/common/extensions/api/media_galleries.idl
@@ -19,7 +19,9 @@ namespace mediaGalleries {
     // before returning the permitted set. Default is silent.  If the value
     // 'yes' is passed, or if the application has not been granted access to
     // any media galleries and the value 'if_needed' is passed, then the
-    // media gallery configuration dialog will be displayed.
+    // media gallery configuration dialog will be displayed. A user
+    // gesture is required for the display of the dialog. (i.e. making the call
+    // in the context of a button press)
     GetMediaFileSystemsInteractivity? interactive;
   };