Changes to SkTDStackNester.

Changes to SkTDStackNester.

SkTDStackNester is a class used by PdfViewer to assist in saving
and restoring the PDF state. Clean up and test this class.

Add some documentation.

Add FIXME's where I have questions to resolve.

Fix a bug where fNestingLevel was not initialized.

Remove a commented out line of code copied over from
SkTDStack.

Rename SkTDStackNester::nests() to nestingLevel() and make it const.

Remove unnecessary predeclaration and friend declaration.

Remove index() (both const and non-const versions). They were
unused, return something that may not be expected (index from
the top, rather than from the bottom), and don't work to get any
elements in earlier Recs once the first one is full.

Report a warning if the nesting level goes above the maximum level,
or if we attempt to bring it below zero.

Prevent fNestingLevel from dropping below zero.

Add kUnusedObject_SkPdfIssue, and use it where appropriate.

Depends on https://codereview.chromium.org/64093009/

R=mtklein@google.com

Committed: https://code.google.com/p/skia/source/detail?r=12328

[scroggo, 2013-11-20 20:04:07]

Right now I am trusting Edison's wisdom in his differences from SkTDStack, but
it might make more sense to make SkTDStackNester its own class that has-a
SkTDStack, and make any necessary optimizations later. What do you think?

[mtklein, 2013-11-20 20:45:13]

On 2013/11/20 20:04:07, scroggo wrote:
> Right now I am trusting Edison's wisdom in his differences from SkTDStack, but
> it might make more sense to make SkTDStackNester its own class that has-a
> SkTDStack, and make any necessary optimizations later. What do you think?

This change LGTM.  Everything here seems to be a strict improvement on the
status quo, though I remain somewhat baffled by that status quo.

My suggestions for next refactorings:
  - implement this with SkTDStack<SkTDStack<T> >, or SkTArray<SkTArray<T> >;
  - if you figure out why calling unnest() is remarkable enough to merit
Edison's TODO and your log-info, I'd love to know;
  - I am also in favor of replacing nest/unnest with save/restore.

[scroggo, 2013-11-20 21:41:07]

Committed patchset #5 manually as r12328 (presubmit successful).