Clean up a few warnings on Windows x64.



git-svn-id: http://skia.googlecode.com/svn/trunk@10233 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/utils/SkThreadUtils_win.cpp b/src/utils/SkThreadUtils_win.cpp
index 208ffde..a064d3b 100644
--- a/src/utils/SkThreadUtils_win.cpp
+++ b/src/utils/SkThreadUtils_win.cpp
@@ -108,7 +108,7 @@
 static unsigned int nth_set_bit(unsigned int n, DWORD_PTR mask) {
     n %= num_bits_set(mask);
     for (unsigned int setBitsSeen = 0, currentBit = 0; true; ++currentBit) {
-        if (mask & (1 << currentBit)) {
+        if (mask & (static_cast<DWORD_PTR>(1) << currentBit)) {
             ++setBitsSeen;
             if (setBitsSeen > n) {
                 return currentBit;