SkClipStack::Element tweaks.

SkClipStack::Element tweaks.

(
This is intended to facilitate efficient kMatrix_SaveFlags emulation
on restore():

  * collect all clip stack elements for the current save count into a
    side clip stack
  * canvas.restore(everything)
  * replay the collected clip ops to restore the initial clip state
  => we restored the matrix but the clip state is unchanged
)

Two main changes:

  * expose the save count for SkClipStack::Element
  * expose a replay method for the same (logic relocated from
    SkCanvas::replayClips)

The SkCanvas::ClipVisitor shuffling is to enable forward decl
in SkClipStack.h (cannot fwdecl a nested class).

R=reed@google.com, robertphillips@google.com
TBR=reed@google.com

Author: fmalita@chromium.org

Review URL: https://codereview.chromium.org/269693003