zygote: Don't CHECK if browser gone during zygote startup

content/zygote/zygote_main_linux.cc

Index: content/zygote/zygote_main_linux.cc
diff --git a/content/zygote/zygote_main_linux.cc b/content/zygote/zygote_main_linux.cc
index b6f7089e35cdfe1d46c9a010e7e9b8b90cc0351a..733d6df231f4a98cb4b49ebe1b99e26f4db26d40 100644
--- a/content/zygote/zygote_main_linux.cc
+++ b/content/zygote/zygote_main_linux.cc
@@ -617,10 +617,16 @@ bool ZygoteMain(
 
   if (using_layer1_sandbox) {
     // Let the ZygoteHost know we're booting up.
-    CHECK(base::UnixDomainSocket::SendMsg(kZygoteSocketPairFd,
-                                          kZygoteBootMessage,
-                                          sizeof(kZygoteBootMessage),
-                                          std::vector<int>()));
+    if (!base::UnixDomainSocket::SendMsg(
+            kZygoteSocketPairFd, kZygoteBootMessage, sizeof(kZygoteBootMessage),
+            std::vector<int>())) {
+      // This is not a CHECK failure because the browser process could either
+      // crash or quickly exit while the zygote is starting. In either case a
+      // zygote crash is not useful. http://crbug.com/692227
+      PLOG(ERROR) << "Failed sending zygote boot message";
+      linux_sandbox->set_exiting_before_initialize_sandbox(true);

[James Cook, 2017/04/27 22:30:23]

If I don't do something like this I hit the CHECK(initialize_sandbox_ran_) in
~SandboxLinux(). SandboxLinux is a singleton cleaned up by the AtExitManager.

Alternately I could _exit(1), but this seemed a little better.

[mdempsky, 2017/04/27 22:50:45]

Since this is security sensitive code, I'd prefer we err on the side of
simplicity and just _exit(1).

[James Cook, 2017/04/28 02:39:48]

Sounds good. Done.

+      return false;
+    }
   }
 
   VLOG(1) << "ZygoteMain: initializing " << fork_delegates.size()