stylo: Fix Y scale computation while decomposing a 3D matrix.

While decomposing a 3D matrix, we should normalize the 2nd row right after the
Y scale computation. However, we accidentally use the length of the 1st row to
do the normalization. This causes the wrong Scale3D function while decomposing,
and then leads to the wrong decomposed 3D matrix.

Here, we correct it by using the right value (the length of the 2nd row).

r=hiro https://bugzilla.mozilla.org/show_bug.cgi?id=1381196

components/style/properties/helpers/animated_properties.mako.rs

@@ -2146,7 +2146,7 @@ fn decompose_3d_matrix(mut matrix: ComputedMatrix) -> Result<MatrixDecomposed3D,
     row[1] = combine(row[1], row[0], 1.0, -skew.0);
 
     // Now, compute Y scale and normalize 2nd row.
-    let row1len = (row[0][0] * row[0][0] + row[0][1] * row[0][1] + row[0][2] * row[0][2]).sqrt();
+    let row1len = (row[1][0] * row[1][0] + row[1][1] * row[1][1] + row[1][2] * row[1][2]).sqrt();
     scale.1 = row1len;
     row[1] = [row[1][0] / row1len, row[1][1] / row1len, row[1][2] / row1len];
     skew.0 /= scale.1;