Small refactoring of layer discovery code

Small refactoring of layer discovery code

This CL removes CollectLayers' reliance on having the top most picture (by removing the unused fPictureID member). This then allows making CollectLayers' API closer to that of SkRecordFillBounds in order to facilitate using them interchangeably.

Committed: https://skia.googlesource.com/skia/+/6beeb8f3cc323dabed0905920a336ed77aa5c46d

[robertphillips, 2014-11-09 16:50:14]

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

[bsalomon, 2014-11-10 16:03:15]

lgtm

[robertphillips, 2014-11-10 16:03:38]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2014-11-10 16:03:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/714533002/1

[commit-bot: I haz the power, 2014-11-10 16:27:32]

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

[commit-bot: I haz the power, 2014-11-10 16:27:33]

Try jobs failed on following builders:
  Build-Win-VS2013-x86-Debug-Trybot on client.skia.compile
(http://build.chromium.org/p/client.skia.compile/builders/Build-Win-VS2013-x86...)

[robertphillips, 2014-11-10 16:44:06]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2014-11-10 16:44:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/714533002/1

[commit-bot: I haz the power, 2014-11-10 16:48:32]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/6beeb8f3cc323dabed0905920a336ed77aa5c46d