Expose API to store and load data usage stats from UI task runner.

components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc

Index: components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc
diff --git a/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc b/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc
index 1052ff66316203c3ecea0b5ab0b361da90ee5892..8a0a1886a618c3550a03eb6416a46a5c408de492 100644
--- a/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc
+++ b/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc
@@ -4,6 +4,7 @@
 
 #include "components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.h"
 
+#include <string>
 #include <vector>
 
 #include "base/basictypes.h"
@@ -368,8 +369,8 @@ class DailyDataSavingUpdate {
 
 DataReductionProxyCompressionStats::DataReductionProxyCompressionStats(
     PrefService* prefs,
-    scoped_refptr<base::SequencedTaskRunner> task_runner,
-    const base::TimeDelta& delay)
+    const scoped_refptr<base::SequencedTaskRunner>& task_runner,
+    base::TimeDelta delay)
     : pref_service_(prefs),
       task_runner_(task_runner),
       delay_(delay),
@@ -385,7 +386,6 @@ DataReductionProxyCompressionStats::~DataReductionProxyCompressionStats() {
   DCHECK(thread_checker_.CalledOnValidThread());
   WritePrefs();
   pref_change_registrar_->RemoveAll();
-  weak_factory_.InvalidateWeakPtrs();
 }
 
 void DataReductionProxyCompressionStats::Init() {
@@ -450,6 +450,8 @@ void DataReductionProxyCompressionStats::UpdateContentLengths(
   SetInt64(data_reduction_proxy::prefs::kHttpOriginalContentLength,
            total_original);
 
+  // TODO(kundaji): Collect data usage metrics broken down by site and store
+  // in |data_usage_store_|.
   RecordContentLengthPrefs(received_content_length, original_content_length,
                            data_reduction_proxy_enabled, request_type,
                            mime_type, base::Time::Now());