Move ForkWithFlags from sandbox/ to base/ and plug it into LaunchProcess.

base/process/process.h

Index: base/process/process.h
diff --git a/base/process/process.h b/base/process/process.h
index ad8f8eec888f2e7c9297a15d0d693f36af0c871e..3047c3f03883e0c84c65fca4bcb7cea21a828ddb 100644
--- a/base/process/process.h
+++ b/base/process/process.h
@@ -112,6 +112,19 @@ class BASE_EXPORT Process {
 #endif
 };
 
+#if defined(OS_LINUX)
+// A wrapper for clone with fork-like behavior, meaning that it returns the
+// child's pid in the parent and 0 in the child. |flags|, |ptid|, and |ctid| are
+// as in the clone system call (the CLONE_VM flag is not supported).
+//
+// This function uses the libc clone wrapper (which updates libc's pid cache)
+// internally, so callers may expect things like getpid() to work correctly
+// after in both the child and parent. An exception is when this code is run
+// under Valgrind. Valgrind does not support the libc clone wrapper, so the libc
+// pid cache may be incorrect after this function is called under Valgrind.
+BASE_EXPORT pid_t ForkWithFlags(unsigned long flags, pid_t* ptid, pid_t* ctid);

[jln (very slow on Chromium), 2015/01/06 00:22:31]

It may be useful to warn about the usual fork() issues, such as the fact that
forking with threads requires a lot of care (and typically doing an execve()
soon).

[rickyz (no longer on Chrome), 2015/01/06 02:45:37]

Done.

+#endif
+
 }  // namespace base
 
 #endif  // BASE_PROCESS_PROCESS_PROCESS_H_