Don't use a global share group for the passthrough command buffer contexts.

gpu/ipc/service/gpu_command_buffer_stub.h

Index: gpu/ipc/service/gpu_command_buffer_stub.h
diff --git a/gpu/ipc/service/gpu_command_buffer_stub.h b/gpu/ipc/service/gpu_command_buffer_stub.h
index 91b8dfe1c36ff3978808b7af5933d2ee276e8c31..343170a74fe329c22cdf7d2503de4238dc8bca93 100644
--- a/gpu/ipc/service/gpu_command_buffer_stub.h
+++ b/gpu/ipc/service/gpu_command_buffer_stub.h
@@ -37,6 +37,10 @@
 #include "ui/gl/gpu_preference.h"
 #include "url/gurl.h"
 
+namespace gl {
+class GLShareGroup;
+}
+
 namespace gpu {
 struct Mailbox;
 struct SyncToken;
@@ -233,6 +237,7 @@ class GPU_EXPORT GpuCommandBufferStub
   std::unique_ptr<CommandExecutor> executor_;
   std::unique_ptr<SyncPointClient> sync_point_client_;
   scoped_refptr<gl::GLSurface> surface_;
+  scoped_refptr<gl::GLShareGroup> share_group_;
 
   base::ObserverList<DestructionObserver> destruction_observers_;