Reland "Use absl::optional instead or rtc::Optional"

This reverts commit 28e6a164bf9d2a42545d058bd50d39e1767f7398.

Reason for revert: the static initializer removed from abseil

Original change's description:
> Revert "Use absl::optional instead or rtc::Optional"
>
> This reverts commit 7ba9e92fa0dfb16579f4f6ecd746397bdfdd174d.
>
> Reason for revert: Breaks Chromium static initialized regression test.
> https://ci.chromium.org/p/chromium/builders/luci.chromium.try/android-marshmallow-arm64-rel/5068
>
> Original change's description:
> > Use absl::optional instead or rtc::Optional
> >
> > BUG: webrtc:9078
> > Change-Id: I69aedce324d86e8894b81210a2de17c5ef68fd11
> > Reviewed-on: https://webrtc-review.googlesource.com/77082
> > Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
> > Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> > Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#23440}
>
> TBR=danilchap@webrtc.org,mbonadei@webrtc.org,kwiberg@webrtc.org
>
> Change-Id: I09ae74bddc69d0b25c8dfbcacc4ec906b34ca748
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://webrtc-review.googlesource.com/79980
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#23449}

TBR=danilchap@webrtc.org,mbonadei@webrtc.org,kwiberg@webrtc.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ib5dc71fb63fe02b78743b03f8252b962616eead0
Bug: webrtc:9078
Reviewed-on: https://webrtc-review.googlesource.com/82760
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23586}
diff --git a/DEPS b/DEPS
index cec6124d..e4ed3a7 100644
--- a/DEPS
+++ b/DEPS
@@ -744,4 +744,7 @@
   "+rtc_base",
   "+test",
   "+rtc_tools",
+
+  # Abseil whitelist.
+  "+absl/types/optional.h",
 ]