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

include/core/SkMatrix.h

Index: include/core/SkMatrix.h
diff --git a/include/core/SkMatrix.h b/include/core/SkMatrix.h
index fa7a63a4e8670fa349a9f007adafe748e4a2bf34..dc482b888ed16bf8eb16d951e4a871a3afdb5d26 100644
--- a/include/core/SkMatrix.h
+++ b/include/core/SkMatrix.h
@@ -723,7 +723,7 @@ private:
      */
     bool isFinite() const;
 
-    static void ComputeInv(SkScalar dst[9], const SkScalar src[9], SkScalar invDet, bool isPersp);
+    static void ComputeInv(SkScalar dst[9], const SkScalar src[9], double invDet, bool isPersp);
 
     void setScaleTranslate(SkScalar sx, SkScalar sy, SkScalar tx, SkScalar ty) {
         fMat[kMScaleX] = sx;