add warning for use-fake-ui-for-media-stream

adds a warning when the --use-fake-ui-for-media-stream flag is used.
This flag is used in automated testing and skips the getUserMedia permissions
prompt. It might be slightly harmful to real users.

BUG=520377

Review-Url: https://codereview.chromium.org/2727143006
Cr-Commit-Position: refs/heads/master@{#457733}

chrome/browser/ui/startup/bad_flags_prompt.cc

@@ -86,6 +86,10 @@ void ShowBadFlagsPrompt(Browser* browser) {
     // if they are not.
     switches::kUnsafelyTreatInsecureOriginAsSecure,
 
+    // This flag allows sites to access the camera and microphone without
+    // getting the user's permission.
+    switches::kUseFakeUIForMediaStream,
+
     NULL
   };