Fix code that left shifts a negative value. This has undefined behavior.

include/core/SkFixed.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkFixed_DEFINED
 #define SkFixed_DEFINED
 
@@ skipping 42 matching lines @@
 #define SkFixedToDouble(x)  ((x) * 1.52587890625e-5)
 #define SkDoubleToFixed(x)  ((SkFixed)((x) * SK_Fixed1))
 
 /** Converts an integer to a SkFixed, asserting that the result does not overflow
     a 32 bit signed integer
 */
 #ifdef SK_DEBUG
     inline SkFixed SkIntToFixed(int n)
     {
         SkASSERT(n >= -32768 && n <= 32767);
-        return n << 16;
+        // Left shifting a negative value has undefined behavior in C, so we cast to unsigned before
+        // shifting.
+        return (unsigned)n << 16;
     }
 #else
-    //  force the cast to SkFixed to ensure that the answer is signed (like the debug version)
-    #define SkIntToFixed(n)     (SkFixed)((n) << 16)
+    // Left shifting a negative value has undefined behavior in C, so we cast to unsigned before
+    // shifting. Then we force the cast to SkFixed to ensure that the answer is signed (like the
+    // debug version).
+    #define SkIntToFixed(n)     (SkFixed)((unsigned)(n) << 16)
 #endif
 
 #define SkFixedRoundToInt(x)    (((x) + SK_FixedHalf) >> 16)
 #define SkFixedCeilToInt(x)     (((x) + SK_Fixed1 - 1) >> 16)
 #define SkFixedFloorToInt(x)    ((x) >> 16)
 
 #define SkFixedRoundToFixed(x)  (((x) + SK_FixedHalf) & 0xFFFF0000)
 #define SkFixedCeilToFixed(x)   (((x) + SK_Fixed1 - 1) & 0xFFFF0000)
 #define SkFixedFloorToFixed(x)  ((x) & 0xFFFF0000)
 
@@ skipping 73 matching lines @@
 #define Sk48Dot16FloorToInt(x)    static_cast<int>((x) >> 16)
 
 static inline float Sk48Dot16ToScalar(Sk48Dot16 x) {
     return static_cast<float>(x * 1.5258789e-5);  // x * (1.0f / (1 << 16))
 }
 #define SkFloatTo48Dot16(x)       (static_cast<Sk48Dot16>((x) * (1 << 16)))
 
 #define SkScalarTo48Dot16(x)      SkFloatTo48Dot16(x)
 
 #endif