Revert "Remove rtc::Optional alias and api:optional target"

This reverts commit 6f5b0f920af08d66e6b77ee4f91ade5797145368.

Reason for revert: Breaks internal project.

Original change's description:
> Remove rtc::Optional alias and api:optional target
> 
> Update left-overs where old target still was used.
> 
> Bug: webrtc:9078
> Change-Id: I2162c928091fc4ff1dea33a3f03adbe47207d206
> Reviewed-on: https://webrtc-review.googlesource.com/84740
> Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#23913}

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

Change-Id: I95f5ec33520b823c3d0c9cb83d945d6a15355367
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9078
Reviewed-on: https://webrtc-review.googlesource.com/88140
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23921}

common_types.h

@@ -17,6 +17,7 @@
 #include <vector>
 
 #include "api/array_view.h"
+#include "api/optional.h"
 // TODO(sprang): Remove this include when all usage includes it directly.
 #include "api/video/video_bitrate_allocation.h"
 #include "rtc_base/checks.h"