Replace std::deque usage in drp_event_store with a circular buffer.

components/data_reduction_proxy/core/common/data_reduction_proxy_event_store.cc

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_event_store.cc
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_event_store.cc b/components/data_reduction_proxy/core/common/data_reduction_proxy_event_store.cc
index df8c57c6b6f86dff1b44d7a3c357f1fea41ae1cc..8f7db517adb5fc1d1c4040a30ef041bcb43012d4 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_event_store.cc
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_event_store.cc
@@ -5,19 +5,16 @@
 #include "components/data_reduction_proxy/core/common/data_reduction_proxy_event_store.h"
 
 #include <stddef.h>
-#include <stdint.h>
+
 #include <utility>
-#include <vector>
 
 #include "base/json/json_writer.h"
-#include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "base/values.h"
-#include "components/data_reduction_proxy/core/common/data_reduction_proxy_event_storage_delegate.h"
 
 namespace {
 
@@ -60,7 +57,7 @@ std::string JoinListValueStrings(base::ListValue* list_value) {
     if (!value->GetAsString(&value_string))
       return std::string();
 
-    values.push_back(value_string);
+    values.push_back(std::move(value_string));
   }
 
   return base::JoinString(values, ";");
@@ -93,10 +90,10 @@ void DataReductionProxyEventStore::AddConstants(
 }
 
 DataReductionProxyEventStore::DataReductionProxyEventStore()
-    : enabled_(false),
+    : oldest_event_index_(0),
+      enabled_(false),
       secure_proxy_check_state_(CHECK_UNKNOWN),
-      expiration_ticks_(0) {
-}
+      expiration_ticks_(0) {}
 
 DataReductionProxyEventStore::~DataReductionProxyEventStore() {
 }
@@ -137,17 +134,29 @@ DataReductionProxyEventStore::GetSummaryValue() const {
   }
 
   auto events_list = base::MakeUnique<base::ListValue>();
-  for (const auto& event : stored_events_)
-    events_list->Append(event->CreateDeepCopy());
+
+  DCHECK(oldest_event_index_ == 0 ||
+         stored_events_.size() == kMaxEventsToStore);
+  for (size_t i = oldest_event_index_; i < stored_events_.size(); ++i)
+    events_list->Append(stored_events_[i]->CreateDeepCopy());
+  for (size_t i = 0; i < oldest_event_index_; ++i)
+    events_list->Append(stored_events_[i]->CreateDeepCopy());
+
   data_reduction_proxy_values->Set("events", std::move(events_list));
   return data_reduction_proxy_values;
 }
 
 void DataReductionProxyEventStore::AddEvent(
     std::unique_ptr<base::Value> event) {
-  if (stored_events_.size() == kMaxEventsToStore)
-    stored_events_.pop_front();
-  stored_events_.push_back(std::move(event));
+  if (stored_events_.size() < kMaxEventsToStore) {
+    stored_events_.push_back(std::move(event));
+    return;
+  }
+  DCHECK_EQ(kMaxEventsToStore, stored_events_.size());
+
+  stored_events_[oldest_event_index_++] = std::move(event);
+  if (oldest_event_index_ >= stored_events_.size())
+    oldest_event_index_ = 0;
 }
 
 void DataReductionProxyEventStore::AddEnabledEvent(