Fix overflow in user traffic content type histogram

components/data_use_measurement/core/data_use_measurement.cc

Index: components/data_use_measurement/core/data_use_measurement.cc
diff --git a/components/data_use_measurement/core/data_use_measurement.cc b/components/data_use_measurement/core/data_use_measurement.cc
index fcebeb8d070d079520f1ebde9a28275fcd20bc27..345eff248ebd54fbe695ae3e4fff42a3d0d5bbaa 100644
--- a/components/data_use_measurement/core/data_use_measurement.cc
+++ b/components/data_use_measurement/core/data_use_measurement.cc
@@ -100,6 +100,8 @@ DataUseMeasurement::DataUseMeasurement(
 {
   DCHECK(ascriber_);
   DCHECK(url_request_classifier_);
+  memset(user_traffic_content_type_bytes_, 0,
+         sizeof(user_traffic_content_type_bytes_));
 
 #if defined(OS_ANDROID)
   int64_t bytes = 0;
@@ -445,12 +447,17 @@ void DataUseMeasurement::RecordContentTypeHistogram(
   // Use the more primitive STATIC_HISTOGRAM_POINTER_BLOCK macro because the
   // simple UMA_HISTOGRAM_ENUMERATION macros don't expose 'AddCount'.
   if (is_user_traffic) {
-    STATIC_HISTOGRAM_POINTER_BLOCK(
-        "DataUse.ContentType.UserTraffic", AddCount(content_type, bytes),
-        base::LinearHistogram::FactoryGet(
-            "DataUse.ContentType.UserTraffic", 1, DataUseUserData::TYPE_MAX,
-            DataUseUserData::TYPE_MAX + 1,
-            base::HistogramBase::kUmaTargetedHistogramFlag));
+    bytes += user_traffic_content_type_bytes_[content_type];
+    if (bytes >= 1024) {
+      STATIC_HISTOGRAM_POINTER_BLOCK(
+          "DataUse.ContentType.UserTrafficKB",
+          AddCount(content_type, bytes / 1024),
+          base::LinearHistogram::FactoryGet(
+              "DataUse.ContentType.UserTrafficKB", 1, DataUseUserData::TYPE_MAX,

[Ilya Sherman, 2017/05/02 21:28:52]

This histogram would still overflow if there are ~4.5 Terabytes of data used
during a single run of Chrome.  Is that level of use implausible?  Chrome's
uptime can be tremendously high: as high as months or years.  Though, I guess we
don't really tend to look at data from versions that are many months old, so
maybe that's not a big concern.

[Ilya Sherman, 2017/05/02 21:29:58]

Actually, slightly less than 4.5 TB, as I was calculating for unsigned ints. 
But, still, "single-digit terabytes" is the right order of magnitude.

+              DataUseUserData::TYPE_MAX + 1,
+              base::HistogramBase::kUmaTargetedHistogramFlag));
+    }
+    user_traffic_content_type_bytes_[content_type] = bytes % 1024;
   } else {
     STATIC_HISTOGRAM_POINTER_BLOCK(
         "DataUse.ContentType.Services", AddCount(content_type, bytes),