Remove -Wno-unguarded-availability.

The code that was causing the problem has been removed from absl.

Removed from:
https://chromium.googlesource.com/external/github.com/abseil/abseil-cpp/+/8f96be6ca60d967bd4b37f93d0a03bcff4145200
---
5aba0b15eaf6c5beff0e91670a7cdf5ad1151886 by Derek Mauro <dmauro@google.com>:

Use std::chrono to get the current time on both Apple and Windows
platforms, eliminating the unnecessarily complicated logic on Apple
platforms.

PiperOrigin-RevId: 206979219
---

Bug: webrtc:9557
Change-Id: Idb2858b39b2ef8ed7e69d4d740faef9d369e8747
Reviewed-on: https://chromium-review.googlesource.com/1165348
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581883}

third_party/abseil-cpp/absl/time/BUILD.gn

@@ -14,21 +14,11 @@ if (build_with_chromium) {
   visibility = [ "*" ]
 }
 
-config("suppress_unguarded_availability") {
-  # TODO(bugs.webrtc.org/9557): Remove -Wno-unguarded-availability when
-  # abseil will support Xcode 9.0+ (it currently supports Xcode 7.3.1+
-  # which doesn't have -Wunguarded-availability and __builtin_available).
-  cflags = [
-    "-Wno-unguarded-availability",
-  ]
-}
-
 source_set("time") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/abseil-cpp:absl_default_cflags_cc",
-    ":suppress_unguarded_availability",
   ]
   public_configs = [ "//third_party/abseil-cpp:absl_include_config" ]
   sources = [