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

gpu/command_buffer/common/capabilities.h

Index: gpu/command_buffer/common/capabilities.h
diff --git a/gpu/command_buffer/common/capabilities.h b/gpu/command_buffer/common/capabilities.h
index 30cedc8d3f0eca6eaf468fb89f3aa0ec260a6ff6..e4ff124b87493806e0f04d1933a380ab547dec7c 100644
--- a/gpu/command_buffer/common/capabilities.h
+++ b/gpu/command_buffer/common/capabilities.h
@@ -78,6 +78,9 @@ struct GPU_EXPORT Capabilities {
   int max_vertex_attribs;
   int max_vertex_texture_image_units;
   int max_vertex_uniform_vectors;
+  // MAX_VIEWPORT_DIMS[2]
+  int max_viewport_width;
+  int max_viewport_height;
   int num_compressed_texture_formats;
   int num_shader_binary_formats;
   int bind_generates_resource_chromium;