Add a useful DCHECK to RestoreCurrentTextureBindings.

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index e1eea99544f79a8640fd96780d5741671a91c3a0..f8a9f9af69597e1f292ee95bc239a804429afa5d 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -2296,6 +2296,7 @@ ScopedGLErrorSuppressor::~ScopedGLErrorSuppressor() {
 }
 
 static void RestoreCurrentTextureBindings(ContextState* state, GLenum target) {
+  DCHECK(!state->texture_units.empty());
   TextureUnit& info = state->texture_units[0];
   GLuint last_id;
   scoped_refptr<TextureRef>& texture_ref = info.GetInfoForTarget(target);