Explicitly friend ::SkPrivateEffectInitializer to flattenables.

Explicitly friend ::SkPrivateEffectInitializer to flattenables.

Looks like the -Wmicrosoft warnings in the bug are coming up specificically
because many the effects in SkLightingImageFilter are defined inside an
anonymous namespace (spanning, I think, lines 33-702 today).

BUG=skia:4091
No public API changes.
TBR=reed@google.com

Committed: https://skia.googlesource.com/skia/+/ab374cf894f136418ad573dfb5da4fcb48f47db0

[mtklein_C, 2015-08-24 18:24:36]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-24 18:24:48]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1311783004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1311783004/1

[commit-bot: I haz the power, 2015-08-24 18:29:50]

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

[commit-bot: I haz the power, 2015-08-24 18:29:50]

Dry run: This issue passed the CQ dry run.

[mtklein_C, 2015-08-24 18:30:37]

mtklein@chromium.org changed reviewers:
+ thakis@chromium.org

[mtklein_C, 2015-08-24 18:30:38]

Agreed that it looks simplest to fix by declaring SkPrivateEffectInitializer and
friending ::that.

[Nico, 2015-08-24 18:36:11]

lgtm, thanks!

[mtklein_C, 2015-08-24 19:32:46]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-08-24 19:32:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1311783004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1311783004/1

[commit-bot: I haz the power, 2015-08-24 19:33:21]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/ab374cf894f136418ad573dfb5da4fcb48f47db0