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 f272fd999af26c7f841be8920a2fd3135d417fb7..2da41a50359c4a590597c030e7100d1389e2e3b8 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"
@@ -302,8 +303,8 @@ 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),
@@ -319,7 +320,6 @@ DataReductionProxyCompressionStats::~DataReductionProxyCompressionStats() {
   DCHECK(thread_checker_.CalledOnValidThread());
   WritePrefs();
   pref_change_registrar_->RemoveAll();
-  weak_factory_.InvalidateWeakPtrs();
 }
 
 void DataReductionProxyCompressionStats::Init() {
@@ -383,6 +383,8 @@ 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,