Animation: Implement DoubleStyleInterpolation in StringKeyframe

LayoutTests/animations/interpolation/flex-interpolation.html

Index: LayoutTests/animations/interpolation/flex-interpolation.html
diff --git a/LayoutTests/animations/interpolation/flex-interpolation.html b/LayoutTests/animations/interpolation/flex-interpolation.html
index 58d3c578618aaad46855e3809c2a6417cdbbe073..f4afb4c849efb8162a6248a592c4fac155dd0052 100644
--- a/LayoutTests/animations/interpolation/flex-interpolation.html
+++ b/LayoutTests/animations/interpolation/flex-interpolation.html
@@ -50,5 +50,29 @@ assertInterpolation({
   {at: 1, is: '1 1 100%'},
   {at: 1.5, is: '1 1 100%'}
 ]);
+assertInterpolation({
+  property: 'flex-shrink',
+  from: '0',
+  to: '1'
+}, [
+  {at: -0.3, is: '0'},
+  {at: 0, is: '0'},
+  {at: 0.4, is: '0'},
+  {at: 0.6, is: '1'},
+  {at: 1, is: '1'},
+  {at: 1.5, is: '1'}
+]);
+assertInterpolation({
+  property: 'flex-grow',
+  from: '0',
+  to: '1'
+}, [
+  {at: -0.3, is: '0'},
+  {at: 0, is: '0'},
+  {at: 0.4, is: '0'},
+  {at: 0.6, is: '1'},
+  {at: 1, is: '1'},
+  {at: 1.5, is: '1'}

[alancutter (OOO until 2018), 2015/02/04 20:24:37]

This is incorrect, these are animatable as non-negative numbers:
http://dev.w3.org/csswg/css-flexbox-1/#flex-grow-property

[jadeg, 2015/02/05 23:37:01]

The CSSValues dont seem to conform to this though, so I added fail cases to the
tests.

+]);
 </script>
 </body>