Does blend mode creating a render surface always happen with isolation group? Can we just make isolation group creating effect node instead of a layer?

Or is it possible to basically say ignore the blend mode on layer if their target shares the same blend mode as the layer?

Actually, I just realized layer's draw blend mode is always set to the default (SkBlendMode::kSrcOver). Whenever we have a non-default blend mode, we create a render surface and the surface does the blending.
So, we can just delete LayerImpl::draw_blend_mode as its always the default. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/815ffdb83a680ac0e348f1305d7051a574f17b20

commit 815ffdb83a680ac0e348f1305d7051a574f17b20
Author: jaydasika <jaydasika@chromium.org>
Date: Mon Apr 10 18:19:46 2017

cc : Delete layer's draw blend mode

Since its always the default. Whenever there is a non-default blend
mode, render surface takes care of blending

BUG= 709269 
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2801383002
Cr-Commit-Position: refs/heads/master@{#463330}

[modify] https://crrev.com/815ffdb83a680ac0e348f1305d7051a574f17b20/cc/layers/layer.cc
[modify] https://crrev.com/815ffdb83a680ac0e348f1305d7051a574f17b20/cc/layers/layer.h
[modify] https://crrev.com/815ffdb83a680ac0e348f1305d7051a574f17b20/cc/layers/layer_impl.cc
[modify] https://crrev.com/815ffdb83a680ac0e348f1305d7051a574f17b20/cc/layers/layer_impl.h
[modify] https://crrev.com/815ffdb83a680ac0e348f1305d7051a574f17b20/cc/layers/video_layer_impl.cc
[modify] https://crrev.com/815ffdb83a680ac0e348f1305d7051a574f17b20/cc/trees/layer_tree_host_common_unittest.cc
[modify] https://crrev.com/815ffdb83a680ac0e348f1305d7051a574f17b20/cc/trees/occlusion_tracker.cc
[modify] https://crrev.com/815ffdb83a680ac0e348f1305d7051a574f17b20/cc/trees/property_tree_builder.cc