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

base/process/launch.h

Index: base/process/launch.h
diff --git a/base/process/launch.h b/base/process/launch.h
index 0450ddf021e0faa7936bf7118951d050e185251b..5908ab61c5abc24d98930c82c487dc4ca4247b75 100644
--- a/base/process/launch.h
+++ b/base/process/launch.h
@@ -13,6 +13,7 @@
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "base/environment.h"
 #include "base/process/process.h"
 #include "base/process/process_handle.h"
@@ -122,6 +123,14 @@ struct BASE_EXPORT LaunchOptions {
   bool allow_new_privs;
 #endif  // defined(OS_LINUX)
 
+#if defined(OS_POSIX)
+  // A callback to be run in immediately prior to executing the new program in
+  // the child process. Warning: If LaunchProcess was called in the presence of
+  // multiple threads, it may not be safe for this callback to perform
+  // allocations or grab locks.
+  Closure pre_exec_hook;

[jln (very slow on Chromium), 2015/01/06 00:43:36]

If we go this route, we should make the warning even stronger. Code between
fork() and exec() needs to be essentially async-signal safe. (And
base::LaunchProcess() is already taking some worrisome liberties). It's also not
obvious how to check that callbacks themselves will stay safe. 

Since, as per our discussion, you only need this to build a synchronization
primitive (so that the child waits for the parent to signal a pipe), perhaps we
should build that in this API instead of giving such a generic API.

I'm not convinced either way, so let's see what mdempsky@ prefers and also get
input from a base/ owner before changing everything.

+#endif
+
 #if defined(OS_CHROMEOS)
   // If non-negative, the specified file descriptor will be set as the launched
   // process' controlling terminal.