linux: implement gpu plugin

gpu/command_buffer/service/gles2_cmd_decoder.h

Index: gpu/command_buffer/service/gles2_cmd_decoder.h
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.h b/gpu/command_buffer/service/gles2_cmd_decoder.h
index b10e69569a494e0fd5c59e6463171568b2d91550..2f248242c0ffe1f4df6ead675fef7b8069270112 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.h
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.h
@@ -8,14 +8,16 @@
 #define GPU_COMMAND_BUFFER_SERVICE_GLES2_CMD_DECODER_H_
 
 #include <build/build_config.h>
-#if defined(OS_LINUX)
-#include "gpu/command_buffer/service/x_utils.h"
-#elif defined(OS_WIN)
+#if defined(OS_WIN)
 #include <windows.h>
 #endif
 #include "gpu/command_buffer/service/common_decoder.h"
 
 namespace gpu {
+// Forward-declared instead of including x_utils.h, because including glx.h
+// causes havok.
+class XWindowWrapper;
+
 namespace gles2 {
 
 // This class implements the AsyncAPIInterface interface, decoding GLES2