Removed pinch viewport scroll offset distribution

The associated change in Blink makes the pinch viewport a proper
ScrollableArea meaning the normal path for synchronizing layer scroll
offsets is used.

This is a 2 sided patch, the other CL:
https://codereview.chromium.org/199253002/

BUG=349941

Review URL: https://codereview.chromium.org/210543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260105 0039d316-1c4b-4281-b951-d872f2087c98
LayerTreeHost::~LayerTreeHost() {
  TRACE_EVENT0("cc", "LayerTreeHost::~LayerTreeHost");

  overhang_ui_resource_.reset();

  if (root_layer_.get())
    root_layer_->SetLayerTreeHost(NULL);

  if (proxy_) {
    DCHECK(proxy_->IsMainThread());
    proxy_->Stop();
  }

  RegisterViewportLayers(NULL, NULL, NULL);

  if (root_layer_.get()) {
    root_layer_ = NULL;
  }
}

LayerTreeHost::~LayerTreeHost() {
  TRACE_EVENT0("cc", "LayerTreeHost::~LayerTreeHost");

  overhang_ui_resource_.reset();

  if (root_layer_.get())
    root_layer_->SetLayerTreeHost(NULL);

  if (proxy_) {
    DCHECK(proxy_->IsMainThread());
    proxy_->Stop();
  }

  RegisterViewportLayers(NULL, NULL, NULL);

  if (root_layer_.get()) {
    root_layer_ = NULL;
  }
}
