Fix C4334 warning about 32-bit shift assigned to 64-bits

src/core/SkVarAlloc.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkVarAlloc.h"
 
 struct SkVarAlloc::Block {
@@ skipping 27 matching lines @@
     while (b) {
         Block* prev = b->prev;
         sk_free(b);
         b = prev;
     }
 }
 
 void SkVarAlloc::makeSpace(size_t bytes) {
     SkASSERT(SkIsAlignPtr(bytes));
 
-    size_t alloc = 1<<fLgSize++;
+    size_t alloc = static_cast<size_t>(1)<<fLgSize++;
     while (alloc < bytes + sizeof(Block)) {
         alloc *= 2;
     }
     fBytesAllocated += alloc;
     fBlock = Block::Alloc(fBlock, alloc);
     fByte = fBlock->data();
     fRemaining = alloc - sizeof(Block);
 }