Convert base::Bind in chrome/browser/captive_portal/

This CL converts base::Bind to base::BindRepeating in the
captive_portal/ folder.

This is a code health CL, no behavior change.

Bug: 1007635
Change-Id: I334a3487d1a253ca0322ee0fd215e446a96b992a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274517
Auto-Submit: Xida Chen <xidachen@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783734}

chrome/browser/captive_portal/captive_portal_browsertest.cc

@@ -439,8 +439,8 @@ CaptivePortalObserver::CaptivePortalObserver(Profile* profile)
           CaptivePortalServiceFactory::GetForProfile(profile)),
       captive_portal_result_(
           captive_portal_service_->last_detection_result()) {
-  subscription_ = captive_portal_service_->RegisterCallback(
-      base::Bind(&CaptivePortalObserver::Observe, base::Unretained(this)));
+  subscription_ = captive_portal_service_->RegisterCallback(base::BindRepeating(
+      &CaptivePortalObserver::Observe, base::Unretained(this)));
 }
 
 void CaptivePortalObserver::WaitForResults(int num_results_to_wait_for) {