Fix some signed overflow errors causing undefined behavior (in theory).

webrtc/base/ipaddress.cc

Index: webrtc/base/ipaddress.cc
diff --git a/webrtc/base/ipaddress.cc b/webrtc/base/ipaddress.cc
index c92f33c74d8baaa9b947da93e7fa1e7099d17982..b1d1265ae479ee4c97f106d2a52fabb0614ca897 100644
--- a/webrtc/base/ipaddress.cc
+++ b/webrtc/base/ipaddress.cc
@@ -412,7 +412,9 @@ int CountIPMaskBits(IPAddress mask) {
   // http://graphics.stanford.edu/~seander/bithacks.html
   // Counts the trailing 0s in the word.
   unsigned int zeroes = 32;
-  word_to_count &= -static_cast<int32_t>(word_to_count);
+  // This could also be written word_to_count &= -word_to_count, but
+  // MSVC emits warning C4146 when negating an unsigned number.
+  word_to_count &= ~word_to_count + 1;  // Isolate lowest set bit.
   if (word_to_count) zeroes--;
   if (word_to_count & 0x0000FFFF) zeroes -= 16;
   if (word_to_count & 0x00FF00FF) zeroes -= 8;
@@ -522,4 +524,4 @@ IPAddress GetAnyIP(int family) {
   return rtc::IPAddress();
 }
 
-}  // Namespace rtc
+}  // namespace rtc