Fix "value possibly truncated" warnings on MSVC, net/spdy/ edition.

net/spdy/spdy_frame_builder.h

Index: net/spdy/spdy_frame_builder.h
diff --git a/net/spdy/spdy_frame_builder.h b/net/spdy/spdy_frame_builder.h
index 7aa6db4b650b7434af9f17a05d64d1af2b874ea2..bd709770c702a7e856f635b2f00c3d6aa2aa7d64 100644
--- a/net/spdy/spdy_frame_builder.h
+++ b/net/spdy/spdy_frame_builder.h
@@ -106,7 +106,7 @@ class NET_EXPORT_PRIVATE SpdyFrameBuilder {
   }
   bool WriteUInt64(uint64 value) {
     uint32 upper = htonl(value >> 32);
-    uint32 lower = htonl(value);
+    uint32 lower = htonl(static_cast<uint32>(value));
     return (WriteBytes(&upper, sizeof(upper)) &&
             WriteBytes(&lower, sizeof(lower)));
   }