Add a StatisticsDeltaReader class and use cases

base/base.gyp

Index: base/base.gyp
diff --git a/base/base.gyp b/base/base.gyp
index d378d141fa07788bb547acafa34b59f211fbb168..98353b003e56c6e66bb194cf470157a987e664b3 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -605,6 +605,7 @@
         'template_util_unittest.cc',
         'test/expectations/expectation_unittest.cc',
         'test/expectations/parser_unittest.cc',
+        'test/histogram_recorder_unittest.cc',
         'test/test_reg_util_win_unittest.cc',
         'test/trace_event_analyzer_unittest.cc',
         'threading/non_thread_safe_unittest.cc',
@@ -871,6 +872,8 @@
         'test/expectations/parser.h',
         'test/gtest_xml_util.cc',
         'test/gtest_xml_util.h',
+        'test/histogram_recorder.cc',
+        'test/histogram_recorder.h',
         'test/launcher/test_launcher.cc',
         'test/launcher/test_launcher.h',
         'test/launcher/test_result.cc',