Fix preparing log file name.

src/log.cc

Index: src/log.cc
diff --git a/src/log.cc b/src/log.cc
index 9861b1b5a68b75961c78a2cebfcd5f655d46220a..a883c4cd3ef3abe206efe0c2e29945e87480db62 100644
--- a/src/log.cc
+++ b/src/log.cc
@@ -1767,8 +1767,16 @@ static void AddIsolateIdIfNeeded(std::ostream& os,  // NOLINT
 
 static void PrepareLogFileName(std::ostream& os,  // NOLINT
                                Isolate* isolate, const char* file_name) {
-  AddIsolateIdIfNeeded(os, isolate);
+  int dir_separator_count = 0;
   for (const char* p = file_name; *p; p++) {
+    if (base::OS::isDirectorySeparator(*p)) dir_separator_count++;
+  }
+
+  for (const char* p = file_name; *p; p++) {
+    if (dir_separator_count == 0) {
+      AddIsolateIdIfNeeded(os, isolate);
+      dir_separator_count--;
+    }
     if (*p == '%') {
       p++;
       switch (*p) {
@@ -1794,6 +1802,7 @@ static void PrepareLogFileName(std::ostream& os,  // NOLINT
           break;
       }
     } else {
+      if (base::OS::isDirectorySeparator(*p)) dir_separator_count--;
       os << *p;
     }
   }