Navigation: Add class ProxyServer to NavigationHandle interface.

This is in reference to a comment on:
https://chromium-review.googlesource.com/c/chromium/src/+/1715283

Change-Id: I7058826e27110d84e9a846394758044b512bac09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1718224Reviewed-by: Lowell Manners <lowell@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Mohamed Abdelhalim <zetamoo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682658}

content/public/browser/navigation_handle.h

@@ -26,6 +26,7 @@ class GURL;
 namespace net {
 class HttpRequestHeaders;
 class HttpResponseHeaders;
+class ProxyServer;
 }  // namespace net
 
 namespace content {