gpu: Fix webview parent compositor crash in context loss

gpu/command_buffer/service/in_process_command_buffer.cc

Index: gpu/command_buffer/service/in_process_command_buffer.cc
diff --git a/gpu/command_buffer/service/in_process_command_buffer.cc b/gpu/command_buffer/service/in_process_command_buffer.cc
index 1d0ce6a8246e4e6c66d448b198d186be02043922..310fcedcccb18385bb1b7c84d57f8ddb545c0f3f 100644
--- a/gpu/command_buffer/service/in_process_command_buffer.cc
+++ b/gpu/command_buffer/service/in_process_command_buffer.cc
@@ -747,7 +747,9 @@ namespace {
 
 void PostCallback(const scoped_refptr<base::MessageLoopProxy>& loop,
                          const base::Closure& callback) {
-  if (!loop->BelongsToCurrentThread()) {
+  // The loop.get() check is to support using InProcessCommandBuffer on a thread
+  // without a message loop.
+  if (loop.get() && !loop->BelongsToCurrentThread()) {
     loop->PostTask(FROM_HERE, callback);
   } else {
     callback.Run();