[fuchsia] Mark fuchsia.web.Contexts' browser processes as critical.

When launching the browser process for a new fuchsia.web.Context, set it
to be the "critical" process for its job. This causes the kernel to
automatically terminate all child processes of the Context, in the event
of the browser process terminating unexpectedly.

Bug: 1109148, fuchsia:56992
Change-Id: Ia4b5d51362039f301609dc9b23a4ffbe48a5d151
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316260
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791331}

fuchsia/engine/context_provider_impl.cc

@@ -517,10 +517,19 @@ void ContextProviderImpl::Create(
         base::JoinString(cors_exempt_headers, ","));
   }
 
-  if (launch_for_test_)
-    launch_for_test_.Run(launch_command, launch_options);
-  else
-    base::LaunchProcess(launch_command, launch_options);
+  base::Process context_process;
+  if (launch_for_test_) {
+    context_process = launch_for_test_.Run(launch_command, launch_options);
+  } else {
+    context_process = base::LaunchProcess(launch_command, launch_options);
+  }
+
+  if (context_process.IsValid()) {
+    // Set |context_process| termination to teardown its job and sub-processes.
+    zx_status_t result = zx_job_set_critical(launch_options.job_handle, 0,
+                                             context_process.Handle());
+    ZX_CHECK(ZX_OK == result, result) << "zx_job_set_critical";
+  }
 
   // |context_request| and any DevTools channels were transferred (not copied)
   // to the Context process.