Cache GL's viewport on the GPU command buffer client side.

gpu/command_buffer/client/client_context_state.h

Index: gpu/command_buffer/client/client_context_state.h
diff --git a/gpu/command_buffer/client/client_context_state.h b/gpu/command_buffer/client/client_context_state.h
index 45cd14b40a850a342889153859af15e761152585..e9f5ad15f3141cef4149dbea80f944be86dbf96b 100644
--- a/gpu/command_buffer/client/client_context_state.h
+++ b/gpu/command_buffer/client/client_context_state.h
@@ -26,9 +26,17 @@ struct GLES2_IMPL_EXPORT ClientContextState {
   // 'changed' will be true if the state was different from 'enabled.
   bool SetCapabilityState(GLenum cap, bool enabled, bool* changed);
 
+  // Cache the user's valid viewport.
+  void SetViewport(GLint x, GLint y, GLsizei width, GLsizei height);
+
   #include "gpu/command_buffer/client/client_context_state_autogen.h"
 
   EnableFlags enable_flags;
+
+  GLint viewport_x = 0;
+  GLint viewport_y = 0;
+  GLsizei viewport_width = 0;
+  GLsizei viewport_height = 0;
 };
 
 }  // namespace gles2