Fix Windows compile.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/619853007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24380 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap/gc-idle-time-handler.cc

@@ -87,7 +87,8 @@ bool GCIdleTimeHandler::DoScavenge(
   // We do not know the allocation throughput before the first Scavenge.
   // TODO(hpayer): Estimate allocation throughput before the first Scavenge.
   if (new_space_allocation_throughput_in_bytes_per_ms == 0) {
-    new_space_allocation_limit = new_space_size * kConservativeTimeRatio;
+    new_space_allocation_limit =
+        static_cast<size_t>(new_space_size * kConservativeTimeRatio);
   } else {
     // We have to trigger scavenge before we reach the end of new space.
     new_space_allocation_limit -=