Fix up signed-integer-overflow warnings

src/core/SkMatrix.cpp

Index: src/core/SkMatrix.cpp
diff --git a/src/core/SkMatrix.cpp b/src/core/SkMatrix.cpp
index b32f372bbdcece567e69ba2c57b8e2d08bf50ff6..6101eb9f9a57f66831abf0f95823ed68ed9799e8 100644
--- a/src/core/SkMatrix.cpp
+++ b/src/core/SkMatrix.cpp
@@ -156,7 +156,7 @@ uint8_t SkMatrix::computeTypeMask() const {
     } else {
         // Only test for scale explicitly if not affine, since affine sets the
         // scale bit.
-        if ((m00 - kScalar1Int) | (m11 - kScalar1Int)) {
+        if ((m00 ^ kScalar1Int) | (m11 ^ kScalar1Int)) {
             mask |= kScale_Mask;
         }