Fix precision error in https://codereview.chromium.org/1188433011/

src/core/SkMatrix.cpp

Index: src/core/SkMatrix.cpp
diff --git a/src/core/SkMatrix.cpp b/src/core/SkMatrix.cpp
index 068902eaace552bfce0e57247ef0de64776db052..b47770a23b2b02968279c0c5b7137d5feecd26df 100644
--- a/src/core/SkMatrix.cpp
+++ b/src/core/SkMatrix.cpp
@@ -786,7 +786,7 @@ bool SkMatrix::asAffine(SkScalar affine[6]) const {
     return true;
 }
 
-void SkMatrix::ComputeInv(SkScalar dst[9], const SkScalar src[9], SkScalar invDet, bool isPersp) {
+void SkMatrix::ComputeInv(SkScalar dst[9], const SkScalar src[9], double invDet, bool isPersp) {
     SkASSERT(src != dst);
     SkASSERT(src && dst);