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::SetMaskLayer(Layer* mask_layer) {
  DCHECK(IsPropertyChangeAllowed());
  if (mask_layer_.get() == mask_layer)
    return;
  if (mask_layer_.get()) {
    DCHECK_EQ(this, mask_layer_->parent());
    mask_layer_->RemoveFromParent();
  }
  mask_layer_ = mask_layer;
  if (mask_layer_.get()) {
    DCHECK(!mask_layer_->parent());
    mask_layer_->RemoveFromParent();
    mask_layer_->SetParent(this);
    mask_layer_->SetIsMask(true);
  }
  SetNeedsFullTreeSync();
}

void Layer::SetMaskLayer(Layer* mask_layer) {
  DCHECK(IsPropertyChangeAllowed());
  if (mask_layer_.get() == mask_layer)
    return;
  if (mask_layer_.get()) {
    DCHECK_EQ(this, mask_layer_->parent());
    mask_layer_->RemoveFromParent();
  }
  mask_layer_ = mask_layer;
  if (mask_layer_.get()) {
    DCHECK(!mask_layer_->parent());
    mask_layer_->RemoveFromParent();
    mask_layer_->SetParent(this);
    mask_layer_->SetIsMask(true);
  }
  SetNeedsFullTreeSync();
}
