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

src/core/SkVarAlloc.cpp

Index: src/core/SkVarAlloc.cpp
diff --git a/src/core/SkVarAlloc.cpp b/src/core/SkVarAlloc.cpp
index 1fbd382c1948d5728462fc02af6b0350886d08de..ea0524b67f24443695e9982d596afc850493c785 100644
--- a/src/core/SkVarAlloc.cpp
+++ b/src/core/SkVarAlloc.cpp
@@ -45,7 +45,7 @@ SkVarAlloc::~SkVarAlloc() {
 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;
     }