Add a missing cast to ValueDeserializer::ReadVarint.

Without this cast, the integer type isn't promoted before being shifted, and so
for types larger than sizeof(int) there is data loss. This will become an issue
once the host begins using this helper to send 64-bit integers.

BUG=chromium:148757

Review-Url: https://codereview.chromium.org/2326653002
Cr-Commit-Position: refs/heads/master@{#39296}

src/value-serializer.cc

@@ -820,7 +820,7 @@ Maybe<T> ValueDeserializer::ReadVarint() {
     if (position_ >= end_) return Nothing<T>();
     uint8_t byte = *position_;
     if (V8_LIKELY(shift < sizeof(T) * 8)) {
-      value |= (byte & 0x7f) << shift;
+      value |= static_cast<T>(byte & 0x7f) << shift;
       shift += 7;
     }
     has_another_byte = byte & 0x80;