Reland of Supervised Users: Create ResourceThrottle only if filtering is enabled

chrome/browser/supervised_user/supervised_user_url_filter.cc

Index: chrome/browser/supervised_user/supervised_user_url_filter.cc
diff --git a/chrome/browser/supervised_user/supervised_user_url_filter.cc b/chrome/browser/supervised_user/supervised_user_url_filter.cc
index 5318b1bf28dbae99b0708f0830cd0f0ab649f8a2..dfece48403fa0ac6a39d594ad43760092df34ab8 100644
--- a/chrome/browser/supervised_user/supervised_user_url_filter.cc
+++ b/chrome/browser/supervised_user/supervised_user_url_filter.cc
@@ -231,7 +231,8 @@ GURL BuildURL(bool is_https, const std::string& host_and_path) {
 }  // namespace
 
 SupervisedUserURLFilter::SupervisedUserURLFilter()
-    : default_behavior_(ALLOW),
+    : enabled_(false),
+      default_behavior_(ALLOW),
       contents_(new Contents()),
       blacklist_(nullptr),
       amp_cache_path_regex_(kAmpCachePathPattern),
@@ -323,6 +324,16 @@ bool SupervisedUserURLFilter::HostMatchesPattern(
   return trimmed_host == trimmed_pattern;
 }
 
+void SupervisedUserURLFilter::SetEnabled(bool enabled) {
+  if (enabled_ == enabled)
+    return;
+
+  enabled_ = enabled;
+
+  for (Observer& observer : observers_)
+    observer.OnSiteListUpdated();
+}
+
 SupervisedUserURLFilter::FilteringBehavior
 SupervisedUserURLFilter::GetFilteringBehaviorForURL(const GURL& url) const {
   supervised_user_error_page::FilteringBehaviorReason reason;
@@ -343,6 +354,11 @@ SupervisedUserURLFilter::GetFilteringBehaviorForURL(
     supervised_user_error_page::FilteringBehaviorReason* reason) const {
   DCHECK(CalledOnValidThread());
 
+  if (!enabled_) {
+    *reason = supervised_user_error_page::DEFAULT;
+    return ALLOW;
+  }
+
   GURL effective_url = GetEmbeddedURL(url);
   if (!effective_url.is_valid())
     effective_url = url;
@@ -645,8 +661,10 @@ GURL SupervisedUserURLFilter::GetEmbeddedURL(const GURL& url) const {
 void SupervisedUserURLFilter::SetContents(std::unique_ptr<Contents> contents) {
   DCHECK(CalledOnValidThread());
   contents_ = std::move(contents);
-  for (Observer& observer : observers_)
-    observer.OnSiteListUpdated();
+  if (enabled_) {
+    for (Observer& observer : observers_)
+      observer.OnSiteListUpdated();
+  }
 }
 
 void SupervisedUserURLFilter::CheckCallback(