Disable sign-compare warnings in Abseil

https://chromium.googlesource.com/chromium/src/+/7721d7fa8cca6e1006137ca5de74c9899aa8ebf3
enabled the warnings by default in Chromium build but Abseil does not like this warning.

Fix confirmed by patching DEPS roll and compiling WebRTC locally.

TBR: mbonadei@chromium.org
Bug: webrtc:9583
Change-Id: If1750a3823d502478cc83cc49e2d8ffbc7ef2e8f
Reviewed-on: https://chromium-review.googlesource.com/1159375
Commit-Queue: Oleh Prypin <oprypin@chromium.org>
Reviewed-by: Marina Ciocea <marinaciocea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580148}

third_party/abseil-cpp/BUILD.gn

@@ -49,6 +49,7 @@ config("absl_default_cflags_cc") {
   if (is_win) {
     cflags_cc += [
       "/wd4005",  # macro-redefinition
+      "/wd4018",  # sign-compare
       "/wd4068",  # unknown pragma
       "/wd4702",  # unreachable code
     ]