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..c12fcd689e035d41d25040b2abfa45195705e694 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;
 
@@ -15,6 +16,19 @@ const char JSONFileValueSerializer::kCannotReadFile[] = "Can't read file.";
 const char JSONFileValueSerializer::kFileLocked[] = "File locked.";
 const char JSONFileValueSerializer::kNoSuchFile[] = "File doesn't exist.";
 
+JSONFileValueSerializer::JSONFileValueSerializer(
+    const base::FilePath& json_file_path,
+    const std::string& histogram_suffix)
+    : json_file_path_(json_file_path),
+      allow_trailing_comma_(false),
+      histogram_suffix_(histogram_suffix) {}
+
+JSONFileValueSerializer::JSONFileValueSerializer(
+    const base::FilePath& json_file_path)
+    : JSONFileValueSerializer(json_file_path, std::string()) {}
+
+JSONFileValueSerializer::~JSONFileValueSerializer() {}
+
 bool JSONFileValueSerializer::Serialize(const base::Value& root) {
   return SerializeInternal(root, false);
 }
@@ -59,6 +73,17 @@ 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.JsonDataReadSizeKilobytes." + histogram_suffix_, 1, 10000, 50,
+        base::HistogramBase::kUmaTargetedHistogramFlag);
+    histogram->Add(static_cast<int>(json_string->size()) / 1024);
+  }
+
   return JSON_NO_ERROR;
 }