Allow external TaskQueue implementations on Linux

webrtc/webrtc.gni

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/arm.gni")
 import("//build/config/features.gni")
@@ skipping 87 matching lines @@
   # https://gcc.gnu.org/wiki/LinkTimeOptimization
   rtc_use_lto = false
 
   # Set to "func", "block", "edge" for coverage generation.
   # At unit test runtime set UBSAN_OPTIONS="coverage=1".
   # It is recommend to set include_examples=0.
   # Use llvm's sancov -html-report for human readable reports.
   # See http://clang.llvm.org/docs/SanitizerCoverage.html .
   rtc_sanitize_coverage = ""
 
+  # Links a default implementation of task queues to targets
+  # that depend on the target rtc_task_queue. Set to false to
+  # use an external implementation.
+  rtc_link_task_queue_impl = true
+
   # Enable libevent task queues on platforms that support it.
+  # rtc_link_task_queue_impl must be set to true for this to
+  # have an effect.
   if (is_win || is_mac || is_ios || is_nacl) {
     rtc_enable_libevent = false
     rtc_build_libevent = false
   } else {
     rtc_enable_libevent = true
     rtc_build_libevent = true
   }
 
   if (current_cpu == "arm" || current_cpu == "arm64") {
     rtc_prefer_fixed_point = true
@@ skipping 188 matching lines @@
                              "public_configs",
                              "suppressed_configs",
                            ])
     configs += invoker.configs
     configs -= rtc_remove_configs
     configs -= invoker.suppressed_configs
     deps = [
       "//build/config:exe_and_shlib_deps",
     ]
     deps += invoker.deps
+
     public_configs = [ rtc_common_inherited_config ]
     if (defined(invoker.public_configs)) {
       public_configs += invoker.public_configs
     }
   }
 }
 
 template("rtc_static_library") {
   static_library(target_name) {
     forward_variables_from(invoker,
@@ skipping 50 matching lines @@
       configs += invoker.configs
       configs -= rtc_remove_configs
       configs -= invoker.suppressed_configs
       public_configs = [ rtc_common_inherited_config ]
       if (defined(invoker.public_configs)) {
         public_configs += invoker.public_configs
       }
     }
   }
 }