Adding grpc::string_ref class.

- Strict subset of
  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3442.html
- Useful to avoid unnecessary string copies.
diff --git a/tools/run_tests/sources_and_headers.json b/tools/run_tests/sources_and_headers.json
index 627e6ac..6154ff0 100644
--- a/tools/run_tests/sources_and_headers.json
+++ b/tools/run_tests/sources_and_headers.json
@@ -1164,6 +1164,21 @@
     ], 
     "headers": [], 
     "language": "c++", 
+    "name": "cxx_string_ref_test", 
+    "src": [
+      "test/cpp/util/string_ref_test.cc"
+    ]
+  }, 
+  {
+    "deps": [
+      "gpr", 
+      "gpr_test_util", 
+      "grpc", 
+      "grpc++", 
+      "grpc_test_util"
+    ], 
+    "headers": [], 
+    "language": "c++", 
     "name": "cxx_time_test", 
     "src": [
       "test/cpp/util/time_test.cc"
@@ -13125,6 +13140,7 @@
       "include/grpc++/status.h", 
       "include/grpc++/status_code_enum.h", 
       "include/grpc++/stream.h", 
+      "include/grpc++/string_ref.h", 
       "include/grpc++/stub_options.h", 
       "include/grpc++/thread_pool_interface.h", 
       "include/grpc++/time.h", 
@@ -13175,6 +13191,7 @@
       "include/grpc++/status.h", 
       "include/grpc++/status_code_enum.h", 
       "include/grpc++/stream.h", 
+      "include/grpc++/string_ref.h", 
       "include/grpc++/stub_options.h", 
       "include/grpc++/thread_pool_interface.h", 
       "include/grpc++/time.h", 
@@ -13213,6 +13230,7 @@
       "src/cpp/util/byte_buffer.cc", 
       "src/cpp/util/slice.cc", 
       "src/cpp/util/status.cc", 
+      "src/cpp/util/string_ref.cc", 
       "src/cpp/util/time.cc"
     ]
   }, 
@@ -13299,6 +13317,7 @@
       "include/grpc++/status.h", 
       "include/grpc++/status_code_enum.h", 
       "include/grpc++/stream.h", 
+      "include/grpc++/string_ref.h", 
       "include/grpc++/stub_options.h", 
       "include/grpc++/thread_pool_interface.h", 
       "include/grpc++/time.h", 
@@ -13346,6 +13365,7 @@
       "include/grpc++/status.h", 
       "include/grpc++/status_code_enum.h", 
       "include/grpc++/stream.h", 
+      "include/grpc++/string_ref.h", 
       "include/grpc++/stub_options.h", 
       "include/grpc++/thread_pool_interface.h", 
       "include/grpc++/time.h", 
@@ -13376,6 +13396,7 @@
       "src/cpp/util/byte_buffer.cc", 
       "src/cpp/util/slice.cc", 
       "src/cpp/util/status.cc", 
+      "src/cpp/util/string_ref.cc", 
       "src/cpp/util/time.cc"
     ]
   },