Add composite() method to InterpolationType interface

Add composite() method to InterpolationType interface

This change refactors composition of InterpolationValues to be handled by
the InterpolationType::composite() virtual method.
This allows different InterpolationTypes to handle composition of their
InterpolationValues in their own way e.g. lists of shadows may force
underlying values to become repeated to a lowest common multiple.

This introduces no change in behaviour.

BUG=437696
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=202408

[alancutter (OOO until 2018), 2015-09-09 05:32:53]

alancutter@chromium.org changed reviewers:
+ dstockwell@chromium.org, shans@chromium.org

[dstockwell, 2015-09-14 00:25:42]

lgtm

[alancutter (OOO until 2018), 2015-09-14 06:02:15]

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

[commit-bot: I haz the power, 2015-09-14 06:02:26]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1322123003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1322123003/1

[commit-bot: I haz the power, 2015-09-14 06:44:23]

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

[commit-bot: I haz the power, 2015-09-14 06:44:24]

Dry run: Try jobs failed on following builders:
  linux_chromium_gn_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[alancutter (OOO until 2018), 2015-09-16 07:53:36]

The CQ bit was checked by alancutter@chromium.org

[commit-bot: I haz the power, 2015-09-16 07:53:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1322123003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1322123003/1

[commit-bot: I haz the power, 2015-09-16 08:04:22]

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

[commit-bot: I haz the power, 2015-09-16 08:04:23]

Try jobs failed on following builders:
  win8_chromium_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win8_chromium_ng/...)

[alancutter (OOO until 2018), 2015-09-16 23:56:08]

The CQ bit was checked by alancutter@chromium.org

[commit-bot: I haz the power, 2015-09-16 23:57:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1322123003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1322123003/1

[commit-bot: I haz the power, 2015-09-17 01:21:12]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=202408