Add rewind capability to SkChunkAlloc

Add rewind capability to SkChunkAlloc

Split off from https://codereview.chromium.org/940533003/ (Decrease GrInOrderDrawBuffer::Cmd's reliance on GrInOrderDrawBuffer)

Committed: https://skia.googlesource.com/skia/+/a9061de9ff8d5b9545e5d1ef7a6e4054e1907c95

[robertphillips, 2015-02-27 15:27:54]

robertphillips@google.com changed reviewers:
+ reed@google.com

[reed1, 2015-02-27 16:19:46]

lgtm

[robertphillips, 2015-02-27 16:20:20]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2015-02-27 16:21:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/967553003/1

[commit-bot: I haz the power, 2015-02-27 16:22:42]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-27 16:22:43]

Try jobs failed on following builders:
  Build-Ubuntu13.10-Clang-x86_64-Debug-Trybot on client.skia.compile
(JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu13.10-Cl...)

[robertphillips, 2015-02-27 16:24:52]

The CQ bit was checked by robertphillips@google.com

[robertphillips, 2015-02-27 16:24:52]

The patchset sent to the CQ was uploaded after l-g-t-m from reed@google.com
Link to the patchset: https://codereview.chromium.org/967553003/#ps20001 (title:
"Fix variable sizes/signs")

[commit-bot: I haz the power, 2015-02-27 16:25:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/967553003/20001

[commit-bot: I haz the power, 2015-02-27 16:32:02]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/a9061de9ff8d5b9545e5d1ef7a6e4054e1907c95