Avoid copies in SkMasks constructor

Avoid copies in SkMasks constructor

Also mark const methods as const.

Committed: https://skia.googlesource.com/skia/+/8b17dcb72cfbfb1e9451a311f927291a48a84d79

[scroggo, 2015-09-25 18:09:04]

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

[msarett, 2015-09-28 00:49:23]

lgtm

[scroggo, 2015-09-28 13:07:51]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2015-09-28 13:08:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1364463005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1364463005/1

[commit-bot: I haz the power, 2015-09-28 13:08:59]

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

[commit-bot: I haz the power, 2015-09-28 13:09:00]

Try jobs failed on following builders:
  skia_presubmit-Trybot on client.skia.fyi (JOB_FAILED,
http://build.chromium.org/p/client.skia.fyi/builders/skia_presubmit-Trybot/bu...)

[scroggo, 2015-09-30 19:15:13]

The CQ bit was checked by scroggo@google.com

[scroggo, 2015-09-30 19:15:13]

The patchset sent to the CQ was uploaded after l-g-t-m from msarett@google.com
Link to the patchset: https://codereview.chromium.org/1364463005/#ps20001
(title: "Rebase")

[commit-bot: I haz the power, 2015-09-30 19:15:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1364463005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1364463005/20001

[commit-bot: I haz the power, 2015-09-30 19:27:04]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/8b17dcb72cfbfb1e9451a311f927291a48a84d79