Reland of Remove blink::WebLayerClient and WebGraphicsLayerDebugInfo.

It is now legal for Blink to provide an implementation of cc::LayerClient.

CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
TBR=danakj@chromium.org

Review URL: https://codereview.chromium.org/1468753002

Cr-Commit-Position: refs/heads/master@{#361195}