Add the ability to run a callback between fork and exec.

base/process/launch_posix.cc

Index: base/process/launch_posix.cc
diff --git a/base/process/launch_posix.cc b/base/process/launch_posix.cc
index c60cfdc9fc430a181f09d904a20b99b70dbe4740..11ec37127905fa33c9614319c895dbfe1c27e1c4 100644
--- a/base/process/launch_posix.cc
+++ b/base/process/launch_posix.cc
@@ -448,6 +448,12 @@ bool LaunchProcess(const std::vector<std::string>& argv,
     }
 #endif
 
+#if defined(OS_POSIX)
+    if (options.pre_exec_delegate != nullptr) {
+      options.pre_exec_delegate->RunAsyncSafe();
+    }
+#endif
+
     for (size_t i = 0; i < argv.size(); i++)
       argv_cstr[i] = const_cast<char*>(argv[i].c_str());
     argv_cstr[argv.size()] = NULL;