Clarify GraphicsContext::beginLayer()/endLayer() have unexpected behaviors.

beginLayer()/endLayer() behaves like save()/restore() for only CTM and clip states.
SkCanvas::saveLayer() behaves the same as save(), but in addition it allocates an offscreen buffer.
It causes the inconsistency for restoring states managed by between SkCanvas and SkPaint.

For example,
    context.translate(1, 0);
    context.setCompositeOperation(CompositeDestinationIn);

    context.beginLayer(1, CompositeSourceIn);
    context.translate(-1, 0);
    context.setCompositeOperation(CompositeSourceOver);
    context.endLayer();

    // At this moment, CTM is translate(1, 0) while current composite operator is CompositeSourceOver.

It's because CTM is managed by SkCanvas, while current composite operator is managed by GraphicsContext.

BUG=423414

Review URL: https://codereview.chromium.org/651243002

git-svn-id: svn://svn.chromium.org/blink/trunk@183902 bbb929c8-8fbe-4397-9dbb-9b2b20218538