[cc_cleanup] remove impl_transform_ variable.

This variable is unused, this can be removed.

Review URL: https://codereview.chromium.org/618693003

Cr-Commit-Position: refs/heads/master@{#297862}

cc/trees/layer_tree_host.h

@@ -400,7 +400,6 @@ class CC_EXPORT LayerTreeHost {
   float page_scale_factor_;
   float min_page_scale_factor_;
   float max_page_scale_factor_;
-  gfx::Transform impl_transform_;
   bool has_gpu_rasterization_trigger_;
   bool content_is_suitable_for_gpu_rasterization_;
   bool gpu_rasterization_histogram_recorded_;