Refactor code for calculating background image geometry

Refactor code for calculating background image geometry

Extract BoxPainter::calculateBackgroundImageGeometry implementation
and related functions to the BackgroundImageGeometry class. This
enables shrinking the API surface of BackgroundImageGeometry and eases
further code health improvements.

BUG=521481
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200850

[davve, 2015-08-19 12:40:36]

davve@opera.com changed reviewers:
+ fs@opera.com, pdr@chromium.org, wangxianzhu@google.com

[davve, 2015-08-19 12:40:37]

BoxPainter::calculateBackgroundImageGeometry wasn't very well
attached to the rest of BoxPainter. Moving it out (along with
helper methods) may make it a bit easier to split up into
digestible pieces.

To ease review of code movements, I do as little changes as
possible except:

 * use an anonymous namespace instead of static functions for the
   helper functions I moved.

 * hide functions of BackgroundImageGeometry.h that no longer
   needs to be exposed as public.

Improvements for the future:

* I don't particularly like that calculate() calls
  setContainerSizeForLayoutObject on the fill layer image.
  Something to move out?

* The whole function is a bit hard to follow. Should be possible
  to factor into more stand-alone pieces.

[fs, 2015-08-19 13:22:17]

lgtm

[davve, 2015-08-19 18:59:02]

The CQ bit was checked by davve@opera.com

[commit-bot: I haz the power, 2015-08-19 18:59:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1300103003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1300103003/1

[commit-bot: I haz the power, 2015-08-19 21:55:34]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200850