Revert of Add additive animation support for CSS property background-position

Revert of Add additive animation support for CSS property background-position (patchset #7 id:120001 of https://codereview.chromium.org/1680803003/ )

Reason for revert:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=animations%2Fbackground-shorthand-crash.html&testType=layout-tests

failing. This sounds related. Reverting.

Original issue's description:
> Add additive animation support for CSS property background-position
> 
> This patch adds support for composite modes on background-position
> CSS animations via element.animate().
> 
> BUG=437696
> 
> Committed: https://crrev.com/38955fb584216cab36eccb20ac2672d38c198df7
> Cr-Commit-Position: refs/heads/master@{#376155}

TBR=dstockwell@chromium.org,ericwilligers@chromium.org,haraken@chromium.org,alancutter@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=437696

[Fady Samuel, 2016-02-18 19:25:55]

Created Revert of Add additive animation support for CSS property
background-position

[commit-bot: I haz the power, 2016-02-18 19:27:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1711923002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1711923002/1

[commit-bot: I haz the power, 2016-02-18 19:30:41]

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

[commit-bot: I haz the power, 2016-02-18 19:30:42]

Failed to apply patch for third_party/WebKit/Source/core/style/ComputedStyle.h:
While running git apply --index -3 -p1;
  error: patch failed: third_party/WebKit/Source/core/style/ComputedStyle.h:140
  error: repository lacks the necessary blob to fall back on 3-way merge.
  error: third_party/WebKit/Source/core/style/ComputedStyle.h: patch does not
apply

Patch:       third_party/WebKit/Source/core/style/ComputedStyle.h
Index: third_party/WebKit/Source/core/style/ComputedStyle.h
diff --git a/third_party/WebKit/Source/core/style/ComputedStyle.h
b/third_party/WebKit/Source/core/style/ComputedStyle.h
index
a500b10215d338bf13665344e012a6f07d7e5dc1..01814ea8ba804c99f9bff1aed255aac9d4d52a23
100644
--- a/third_party/WebKit/Source/core/style/ComputedStyle.h
+++ b/third_party/WebKit/Source/core/style/ComputedStyle.h
@@ -140,7 +140,6 @@
     friend class CachedUAStyle; // Saves Border/Background information for
later comparison.
     friend class ColorPropertyFunctions; // Reads initial style values and
accesses visited and unvisited colors.
     friend class LengthPropertyFunctions; // Reads initial style values.
-    friend class LengthListPropertyFunctions; // Reads initial style values.
     friend class NumberPropertyFunctions; // Reads initial style values.
     friend class PaintPropertyFunctions; // Reads initial style values.