Soften the history.{push,replace}State throttling mechanism.

johnme@ suggests a better way to handle it; let's do that.

Bug: 786211
Change-Id: I446c70117451d2ef49ce13f5884ebb858d9ee392
Reviewed-on: https://chromium-review.googlesource.com/1161400
Commit-Queue: Chris Palmer <palmer@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580455}

third_party/blink/renderer/core/frame/history.cc

@@ -148,22 +148,28 @@ bool History::ShouldThrottleStateObjectChanges() {
   if (!GetFrame()->GetSettings()->GetShouldThrottlePushState())
     return false;
 
-  const int kStateUpdateLimit = 50;
-
-  if (state_flood_guard.count > kStateUpdateLimit) {
-    static constexpr auto kStateUpdateLimitResetInterval =
-        TimeDelta::FromSeconds(10);
-    const auto now = CurrentTimeTicks();
-    if (now - state_flood_guard.last_updated > kStateUpdateLimitResetInterval) {
-      state_flood_guard.count = 0;
-      state_flood_guard.last_updated = now;
-      return false;
-    }
-    return true;
+  if (state_flood_guard.tokens > 0) {
+    state_flood_guard.tokens--;
+    return false;
+  }
+
+  const auto now = TimeTicks::Now();
+  const TimeDelta elapsed = now - state_flood_guard.last_token_grant;
+  static constexpr base::TimeDelta kTimePerToken =
+      base::TimeDelta::FromMilliseconds(16);
+  static constexpr int kMaxTokens = 50;
+  // It is OK to truncate from int64_t to int here.
+  const int tokens_earned = std::min<int>(elapsed / kTimePerToken, kMaxTokens);
+
+  if (tokens_earned > 0) {
+    state_flood_guard.tokens = tokens_earned - 1;  // One consumed immediately.
+    state_flood_guard.last_token_grant = now;
+    return false;
   }
 
-  state_flood_guard.count++;
-  return false;
+  // Drop this event (though ideally it would be delivered once there are tokens
+  // again, unless obsoleted by a subsequent state object change).
+  return true;
 }
 
 bool History::stateChanged() const {

third_party/blink/renderer/core/frame/history.h

@@ -107,8 +107,8 @@ class CORE_EXPORT History final : public ScriptWrappable,
 
   scoped_refptr<SerializedScriptValue> last_state_object_requested_;
   struct {
-    int count;
-    TimeTicks last_updated;
+    int tokens = 0;
+    TimeTicks last_token_grant;
   } state_flood_guard;
 };