Do not persist HttpServerProperties to disk that often.

Add some heuristics to SetAlternativeServices that prevents a
ScheduleUpdatePrefsOnNetworkThread() call after each incoming Alt-Svc header,
based on the time of the last update, and the expiration times of the current
and new in-memory entries.  (Note that the current in-memory entry might be
different from the entry last persisted to disk.)

Repeated requests to the same server typically result in identical Alt-Svc
announcements, which become different when the max-age field (in seconds from
now) gets converted into a base::Time expiration time.  Alternative services do
not need to be persisted to disk with every incoming Alt-Svc header if other
fields (scheme, host, port) are identical.

BUG=554643

Review-Url: https://codereview.chromium.org/2171743002
Cr-Commit-Position: refs/heads/master@{#408273}

net/http/http_server_properties.h

@@ -265,7 +265,8 @@ class NET_EXPORT HttpServerProperties {
   // Set a single alternative service for |origin|.  Previous alternative
   // services for |origin| are discarded.
   // |alternative_service.host| may be empty.
-  // Return true if |alternative_service_map_| is changed.
+  // Return true if |alternative_service_map_| has changed significantly enough
+  // that it should be persisted to disk.
   virtual bool SetAlternativeService(
       const url::SchemeHostPort& origin,
       const AlternativeService& alternative_service,
@@ -274,7 +275,9 @@ class NET_EXPORT HttpServerProperties {
   // Set alternative services for |origin|.  Previous alternative services for
   // |origin| are discarded.
   // Hostnames in |alternative_service_info_vector| may be empty.
-  // Return true if |alternative_service_map_| is changed.
+  // |alternative_service_info_vector| may be empty.
+  // Return true if |alternative_service_map_| has changed significantly enough
+  // that it should be persisted to disk.
   virtual bool SetAlternativeServices(
       const url::SchemeHostPort& origin,
       const AlternativeServiceInfoVector& alternative_service_info_vector) = 0;

net/http/http_server_properties_impl.cc

@@ -413,8 +413,27 @@ bool HttpServerPropertiesImpl::SetAlternativeServices(
   if (it != alternative_service_map_.end()) {
     DCHECK(!it->second.empty());
     if (it->second.size() == alternative_service_info_vector.size()) {
-      changed = !std::equal(it->second.begin(), it->second.end(),
-                            alternative_service_info_vector.begin());
+      const base::Time now = base::Time::Now();
+      changed = false;
+      auto new_it = alternative_service_info_vector.begin();
+      for (const auto& old : it->second) {
+        // Persist to disk immediately if new entry has different scheme, host,
+        // or port.
+        if (old.alternative_service != new_it->alternative_service) {
+          changed = true;
+          break;
+        }
+        // Also persist to disk if new expiration it more that twice as far or
+        // less than half as far in the future.
+        base::Time old_time = old.expiration;
+        base::Time new_time = new_it->expiration;
+        if (new_time - now > 2 * (old_time - now) ||
+            2 * (new_time - now) < (old_time - now)) {
+          changed = true;
+          break;
+        }
+        ++new_it;
+      }
     }
   }