Log pref file size histogram on read rather than on write.

base/json/json_file_value_serializer.cc

Index: base/json/json_file_value_serializer.cc
diff --git a/base/json/json_file_value_serializer.cc b/base/json/json_file_value_serializer.cc
index d60f800cfcfbb9235fc62fc050feb98c87a54740..cdcefef649d8801040a70a57acbd6ff7293964c4 100644
--- a/base/json/json_file_value_serializer.cc
+++ b/base/json/json_file_value_serializer.cc
@@ -7,6 +7,7 @@
 #include "base/files/file_util.h"
 #include "base/json/json_string_value_serializer.h"
 #include "base/logging.h"
+#include "base/metrics/histogram.h"
 
 using base::FilePath;
 
@@ -59,6 +60,21 @@ int JSONFileValueSerializer::ReadFileToString(std::string* json_string) {
     else
       return JSON_CANNOT_READ_FILE;
   }
+
+  if (!histogram_suffix_.empty()) {
+    // The histogram below is an expansion of the UMA_HISTOGRAM_CUSTOM_COUNTS
+    // macro adapted to allow for a dynamically suffixed histogram name.
+    // Note: The factory creates and owns the histogram.
+    base::HistogramBase* histogram =
+        base::Histogram::FactoryGet(
+            "Settings.JsonDataSizeKilobytes." + histogram_suffix_,

[Alexei Svitkine (slow), 2015/01/30 13:49:03]

Should be JsonDataReadSizeKilobytes.

[gab, 2015/01/30 17:33:37]

Oops, done.

+            1,
+            10000,
+            50,
+            base::HistogramBase::kUmaTargetedHistogramFlag);
+    histogram->Add(static_cast<int>(json_string->size()) / 1024);
+  }
+
   return JSON_NO_ERROR;
 }