Update style guide for absl::make_unique.

No-Try: True
Bug: webrtc:10945
Change-Id: I707aefda5d5b224d78b97ce3122e095c7b9b1f1c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153356
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29216}

abseil-in-webrtc.md

@@ -12,7 +12,7 @@ adds the first use.
 ## **Allowed**
 
 * `absl::InlinedVector`
-* `absl::make_unique` and `absl::WrapUnique`
+* `absl::WrapUnique`
 * `absl::optional` and related stuff from `absl/types/optional.h`.
 * `absl::string_view`
 * The functions in `absl/strings/ascii.h`, `absl/strings/match.h`,
@@ -28,6 +28,10 @@ adds the first use.
 
 ## **Disallowed**
 
+### `absl::make_unique`
+
+*Use `std::make_unique` instead.*
+
 ### `absl::Mutex`
 
 *Use `rtc::CriticalSection` instead.*