[cc_cleanup] remove trigger_idle_updates_ variable

This is used earlier in CompositeAndReadback.
Its not used any where now, so removing this.

BUG=

Review URL: https://codereview.chromium.org/604903003

Cr-Commit-Position: refs/heads/master@{#296790}

cc/trees/layer_tree_host.h

@@ -401,7 +401,6 @@ class CC_EXPORT LayerTreeHost {
   float min_page_scale_factor_;
   float max_page_scale_factor_;
   gfx::Transform impl_transform_;
-  bool trigger_idle_updates_;
   bool has_gpu_rasterization_trigger_;
   bool content_is_suitable_for_gpu_rasterization_;
   bool gpu_rasterization_histogram_recorded_;