Update CountDownLatchImpl to Match Specifications

This CL changes the constructor for CountDownLatchImpl to use a DCHECK
to disallow initialization counts less than 0. An initialization count
of exactly 0 is still allowed, in which case countDown() will never
Signal() its WaitableEvent, and all calls to Await() will immediately
return.

Bug: 861813
Change-Id: Ic7807544c792be010f46ca58391a02f64ab1638e
Reviewed-on: https://chromium-review.googlesource.com/1141101
Commit-Queue: Kyle Qian <kyleqian@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576144}

chromeos/components/nearby/count_down_latch_impl.cc

@@ -13,30 +13,31 @@ namespace nearby {
 CountDownLatchImpl::CountDownLatchImpl(int32_t count)
     : count_(count),
       count_waitable_event_(
-          base::WaitableEvent::ResetPolicy::MANUAL /* reset_policy */,
-          count <= 0 ? base::WaitableEvent::InitialState::SIGNALED
-                     : base::WaitableEvent::InitialState::
-                           NOT_SIGNALED /* initial_state*/) {}
+          base::WaitableEvent::ResetPolicy::MANUAL,
+          count_.IsZero() ? base::WaitableEvent::InitialState::SIGNALED
+                          : base::WaitableEvent::InitialState::NOT_SIGNALED) {
+  DCHECK_GE(count, 0);
+}
 
 CountDownLatchImpl::~CountDownLatchImpl() = default;
 
 location::nearby::Exception::Value CountDownLatchImpl::await() {
-  if (count_waitable_event_.IsSignaled())
-    return location::nearby::Exception::NONE;
+  if (!count_waitable_event_.IsSignaled())
+    count_waitable_event_.Wait();
 
-  count_waitable_event_.Wait();
   return location::nearby::Exception::NONE;
 }
 
 location::nearby::ExceptionOr<bool> CountDownLatchImpl::await(
     int32_t timeout_millis) {
-  if (count_waitable_event_.IsSignaled())
-    return location::nearby::ExceptionOr<bool>(true);
-
-  // Return true if |count_waitable_event_| is signaled before it times out.
-  // Otherwise, this returns false due to timing out.
-  return location::nearby::ExceptionOr<bool>(count_waitable_event_.TimedWait(
-      base::TimeDelta::FromMilliseconds(static_cast<int64_t>(timeout_millis))));
+  if (!count_waitable_event_.IsSignaled())
+    // Return true if |count_waitable_event_| is signaled before TimedAwait()
+    // times out. Otherwise, this returns false due to timing out.
+    return location::nearby::ExceptionOr<bool>(
+        count_waitable_event_.TimedWait(base::TimeDelta::FromMilliseconds(
+            static_cast<int64_t>(timeout_millis))));
+
+  return location::nearby::ExceptionOr<bool>(true);
 }
 
 void CountDownLatchImpl::countDown() {