Have each SandboxedProcessLauncherDelegate maintain a zygote.

content/browser/browser_main_loop.cc

Index: content/browser/browser_main_loop.cc
diff --git a/content/browser/browser_main_loop.cc b/content/browser/browser_main_loop.cc
index b5745961efc4ecf17bd2bd6cd5883a492d5be3da..64206e090c339268a4daf87d5476757c52b1fad3 100644
--- a/content/browser/browser_main_loop.cc
+++ b/content/browser/browser_main_loop.cc
@@ -1034,10 +1034,6 @@ void BrowserMainLoop::ShutdownThreadsAndCleanUp() {
   }
 #endif
 
-#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_ANDROID)
-  ZygoteHostImpl::GetInstance()->TearDownAfterLastChild();

[mdempsky, 2015/12/22 21:21:33]

You might try landing just this removal first/separately.  I think we've
established that it's okay to remove and the trybots seem happy with it, but I
can imagine some weird CrOS or non-trybot tests might start complaining.  (And
if so, better to just have to revert/re-revert a simple one-line CL than your
entire CL.)

[Greg K, 2016/01/05 21:42:13]

Done.

-#endif  // defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_ANDROID)
-
   // The device monitors are using |system_monitor_| as dependency, so delete
   // them before |system_monitor_| goes away.
   // On Mac and windows, the monitor needs to be destroyed on the same thread