[mojo] Don't CHECK fail when a Mojo channel isn't available for a child process.

content/child/child_thread_impl.cc

Index: content/child/child_thread_impl.cc
diff --git a/content/child/child_thread_impl.cc b/content/child/child_thread_impl.cc
index 173fb08441cf43f5c35897370cdbed344a2e8acd..afbdfad79f21ca8ec72f15f12781c8eb0f3f9521 100644
--- a/content/child/child_thread_impl.cc
+++ b/content/child/child_thread_impl.cc
@@ -236,7 +236,10 @@ void InitializeMojoIPCChannel() {
   platform_channel.reset(mojo::edk::PlatformHandle(
       base::GlobalDescriptors::GetInstance()->Get(kMojoIPCChannel)));
 #endif
-  CHECK(platform_channel.is_valid());
+  // Mojo isn't supported on all child process types.
+  // TODO(crbug.com/604282): Support Mojo in the remaining processes.
+  if (!platform_channel.is_valid())
+    return;
   mojo::edk::SetParentPipeHandle(std::move(platform_channel));
 }