Fix blendToIdentity case in PerspectiveTransformOperation::blend

LayoutTests/animations/3d/transform-perspective.html

Index: LayoutTests/animations/3d/transform-perspective.html
diff --git a/LayoutTests/animations/3d/transform-perspective.html b/LayoutTests/animations/3d/transform-perspective.html
index 768c120fbdea199c39c8b8b835af66c056fb0a1f..59f9f5464e1f6e769be82a6afbf1d36ba21820eb 100644
--- a/LayoutTests/animations/3d/transform-perspective.html
+++ b/LayoutTests/animations/3d/transform-perspective.html
@@ -32,7 +32,7 @@
     const expectedValues = [
       // [time, element-id, property, expected-value, tolerance]
       [0.5, "box",  "webkitTransform", [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, -0.0075, 0, 0, 0, 1], 0.002],
-      [0.5, "box2", "webkitTransform", [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, -0.00199, 0, 0, 0, 1], 0.002],
+      [0.5, "box2", "webkitTransform", [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, -0.0005, 0, 0, 0, 1], 0.002],

[Julien - ping for review, 2014/07/08 15:09:19]

It seems really weird to me that the current test case is wrong and that we can
modify the expected result without changing any code.

[a.renevier, 2015/04/02 00:14:27]

First, sorry for the really late reply. It seems like the testcase is indeed
wrong. We are going from perspective(1000px) to perspective: none; That is
equivalent to perspective: infinity.

With the formula p = p / (1 - progress), that means that at 0.5, we want the
equivalent of perspective(2000px), which would give us a value of 1/2000 in the
transform matrix. This test would also 

The current expected value is equivalent to perspective(500px)

     ];
     
     function setupTest()