Remove dependency on webrtc optional target

So that webrtc/api:optional target can be removed.

webrtc version of Optional is gone in favor of absl::optional
Chromium already updated not to use rtc::Optional and
use absl::optional only when is needed to be set in webrtc structures.


Bug: webrtc:9078
Change-Id: I282197cb800e46ece9486bb6b6068d1680a40d2b
Reviewed-on: https://chromium-review.googlesource.com/1109830Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Danil Chapovalov <danilchap@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569266}

content/renderer/BUILD.gn

@@ -737,7 +737,6 @@ target(link_target_type, "renderer") {
     "//third_party/opus",
     "//third_party/webrtc/api:libjingle_logging_api",
     "//third_party/webrtc/api:libjingle_peerconnection_api",
-    "//third_party/webrtc/api:optional",
     "//third_party/webrtc/api:rtc_stats_api",
     "//third_party/webrtc/api/audio:aec3_factory",
     "//third_party/webrtc/api/audio_codecs:audio_codecs_api",

third_party/blink/public/BUILD.gn

@@ -584,7 +584,6 @@ source_set("blink_headers") {
     "//skia",
     "//third_party/blink/public/common",
     "//third_party/webrtc/api:libjingle_peerconnection_api",
-    "//third_party/webrtc/api:optional",
     "//third_party/webrtc/rtc_base:rtc_base",
     "//url",
   ]