third_party/protobuf: update to upstream r371

third_party/protobuf/src/google/protobuf/compiler/importer.cc

Index: third_party/protobuf/src/google/protobuf/compiler/importer.cc
diff --git a/third_party/protobuf/src/google/protobuf/compiler/importer.cc b/third_party/protobuf/src/google/protobuf/compiler/importer.cc
index 7689ce9351efbab447d93f79db7df08d599b37c6..422f759f2be75b21881f3591cef5cbd525b07a5b 100644
--- a/third_party/protobuf/src/google/protobuf/compiler/importer.cc
+++ b/third_party/protobuf/src/google/protobuf/compiler/importer.cc
@@ -231,7 +231,12 @@ static string CanonicalizePath(string path) {
   // The Win32 API accepts forward slashes as a path delimiter even though
   // backslashes are standard.  Let's avoid confusion and use only forward
   // slashes.
-  path = StringReplace(path, "\\", "/", true);
+  if (HasPrefixString(path, "\\\\")) {
+    // Avoid converting two leading backslashes.
+    path = "\\\\" + StringReplace(path.substr(2), "\\", "/", true);
+  } else {
+    path = StringReplace(path, "\\", "/", true);
+  }
 #endif
 
   vector<string> parts;