Fix memory leak caused by incorrect usage of map

This CL is fixing an memory leak detected by memlog.

The bug got introduced here:
 https://chromium-review.googlesource.com/c/chromium/src/+/1526067

The loopup in the map is using the [] operator which is creating a
default entry when the key is not present. Over the time, that map
came really big and consumes too much memory.

The fix is to not use the [] operator, but to validate the presence
of the key.

R=rockot@chromium.org,siggi@chromium.org
CC=yuzus@chromium.org

Bug: 973200
Change-Id: I701e682deddace850d094c68d75adea1cec391cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669101Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671407}

ipc/ipc_channel_proxy.cc

@@ -348,8 +348,7 @@ void ChannelProxy::Context::AddListenerTaskRunner(
 void ChannelProxy::Context::RemoveListenerTaskRunner(int32_t routing_id) {
   DCHECK(default_listener_task_runner_->BelongsToCurrentThread());
   base::AutoLock lock(listener_thread_task_runners_lock_);
-  if (base::Contains(listener_thread_task_runners_, routing_id))
-    listener_thread_task_runners_.erase(routing_id);
+  listener_thread_task_runners_.erase(routing_id);
 }
 
 // Called on the IPC::Channel thread.
@@ -360,10 +359,11 @@ base::SingleThreadTaskRunner* ChannelProxy::Context::GetTaskRunner(
     return default_listener_task_runner_.get();
 
   base::AutoLock lock(listener_thread_task_runners_lock_);
-  base::SingleThreadTaskRunner* task_runner =
-      listener_thread_task_runners_[routing_id].get();
-  if (task_runner)
-    return task_runner;
+  auto task_runner = listener_thread_task_runners_.find(routing_id);
+  if (task_runner != listener_thread_task_runners_.end()) {
+    DCHECK(task_runner->second.get());
+    return task_runner->second.get();
+  }
   return default_listener_task_runner_.get();
 }