Implement the V2 sandbox in the process launcher.

content/common/sandbox_init_mac.cc

Index: content/common/sandbox_init_mac.cc
diff --git a/content/common/sandbox_init_mac.cc b/content/common/sandbox_init_mac.cc
index b63600becf012d364ea8284cae2e19c2b1b0c3a3..f73e85db4d109045820688b28b0924360a07169b 100644
--- a/content/common/sandbox_init_mac.cc
+++ b/content/common/sandbox_init_mac.cc
@@ -37,8 +37,11 @@ bool GetSandboxTypeFromCommandLine(int* sandbox_type,
       *base::CommandLine::ForCurrentProcess();
   if (command_line.HasSwitch(switches::kNoSandbox))
     return false;
-  if (command_line.HasSwitch(switches::kV2SandboxedEnabled))
+  if (command_line.HasSwitch(switches::kV2SandboxedEnabled)) {
     CHECK(sandbox::Seatbelt::IsSandboxed());
+    // Do not enable the sandbox if V2 is already enabled.
+    return false;
+  }
 
   std::string process_type =
       command_line.GetSwitchValueASCII(switches::kProcessType);