[Fuchsia] Update FIDL to use |bytes| syntax.

Recent updates to fidlc added a |bytes| synonym for the |vector<uint8>|
type, to improve readability of protocols which need to handle sequences
of octets that are not necessarily UTF-8 encoded (e.g. binary data or
strings using other character encodings).

Change-Id: I3a25086db9ffd5fffe6622b58e7cbd07364531aa
Reviewed-on: https://chromium-review.googlesource.com/c/1476410
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633770}

fuchsia/fidl/web/navigation_controller.fidl

@@ -39,7 +39,7 @@ struct LoadUrlParams {
   bool user_activated = false;
 
   /// Custom HTTP headers.
-  vector<vector<uint8>> headers;
+  vector<bytes> headers;
 };
 
 /// Characterizes the origin of a LoadUrl request.