Don't crash the renderer if the plugin crashes.

ppapi/proxy/proxy_channel.cc

Index: ppapi/proxy/proxy_channel.cc
diff --git a/ppapi/proxy/proxy_channel.cc b/ppapi/proxy/proxy_channel.cc
index f5743e832ec9d0db462482c66cbaf7a65303fb87..2a68def3b1a710bb5cf14c60a8444015f159bc97 100644
--- a/ppapi/proxy/proxy_channel.cc
+++ b/ppapi/proxy/proxy_channel.cc
@@ -50,6 +50,9 @@ int ProxyChannel::TakeRendererFD() {
 IPC::PlatformFileForTransit ProxyChannel::ShareHandleWithRemote(
       base::PlatformFile handle,
       bool should_close_source) {
+  // Channel could be closed if the plugin crashes.
+  if (!channel_.get())
+    return IPC::InvalidPlatformFileForTransit();
   return delegate_->ShareHandleWithRemote(handle, *channel_,
                                           should_close_source);
 }