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
void Layer::ReplaceChild(Layer* reference, scoped_refptr<Layer> new_layer) {
  DCHECK(reference);
  DCHECK_EQ(reference->parent(), this);
  DCHECK(IsPropertyChangeAllowed());

  if (reference == new_layer.get())
    return;

  int reference_index = IndexOfChild(reference);
  if (reference_index == -1) {
    NOTREACHED();
    return;
  }

  reference->RemoveFromParent();

  if (new_layer.get()) {
    new_layer->RemoveFromParent();
    InsertChild(new_layer, reference_index);
  }
}

void Layer::ReplaceChild(Layer* reference, scoped_refptr<Layer> new_layer) {
  DCHECK(reference);
  DCHECK_EQ(reference->parent(), this);
  DCHECK(IsPropertyChangeAllowed());

  if (reference == new_layer.get())
    return;

  int reference_index = IndexOfChild(reference);
  if (reference_index == -1) {
    NOTREACHED();
    return;
  }

  reference->RemoveFromParent();

  if (new_layer.get()) {
    new_layer->RemoveFromParent();
    InsertChild(new_layer, reference_index);
  }
}
