SkCodec::onGetScanlineDecoder must call rewind.

SkCodec::onGetScanlineDecoder must call rewind.

This mirrors the behavior in onGetPixels, and allows the implementation
to share code for handling calls to rewindIfNeeded.

This also fixes a bug where getScanlineDecoder was calling
rewindIfNeeded and treating the result as a bool.

In SkPngCodec, factor out the code to call rewindIfNeeded, and call it
in both onGetPixels and onGetScanlineDecoder.

Update the test to include testing the scanline decoder. Rename "gen"
to "codec" now that it must be an SkCodec.

BUG=skia:3257
Depends on https://codereview.chromium.org/1048423003/ (DIFFERENT ISSUE).

Committed: https://skia.googlesource.com/skia/+/58421544808562973ad66b2e3b8f9da1c5b8a36f

[scroggo, 2015-04-01 13:39:06]

scroggo@google.com changed reviewers:
+ djsollen@google.com, msarett@google.com

[msarett, 2015-04-01 14:23:25]

lgtm

[djsollen, 2015-04-01 16:08:39]

lgtm

[scroggo, 2015-04-01 17:23:57]

The CQ bit was checked by scroggo@google.com

[scroggo, 2015-04-01 17:23:57]

The patchset sent to the CQ was uploaded after l-g-t-m from msarett@google.com,
djsollen@google.com
Link to the patchset: https://codereview.chromium.org/1050893002/#ps20001
(title: "Rebase")

[commit-bot: I haz the power, 2015-04-01 17:24:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1050893002/20001

[commit-bot: I haz the power, 2015-04-01 17:25:29]

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

[commit-bot: I haz the power, 2015-04-01 17:25:30]

Try jobs failed on following builders:
  Build-Ubuntu-Clang-x86_64-Debug-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu-Clang-x...)
  Build-Ubuntu-GCC-Arm7-Debug-Android-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu-GCC-Arm...)

[scroggo, 2015-04-01 17:54:21]

The CQ bit was checked by scroggo@google.com

[scroggo, 2015-04-01 17:54:21]

The patchset sent to the CQ was uploaded after l-g-t-m from msarett@google.com,
djsollen@google.com
Link to the patchset: https://codereview.chromium.org/1050893002/#ps60001
(title: "All codecs can now rewind. Remove code to support unrewindable
codecs.")

[commit-bot: I haz the power, 2015-04-01 17:54:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1050893002/60001

[commit-bot: I haz the power, 2015-04-01 18:25:24]

Committed patchset #4 (id:60001) as
https://skia.googlesource.com/skia/+/58421544808562973ad66b2e3b8f9da1c5b8a36f