arc: Remove a stray 'explicit'

This change addresses a comment that was added just before committing.

Bug: None
Test: git cl try
Change-Id: If6d61b603b6fcaaad18ccaca795ba024c5e84c21
Reviewed-on: https://chromium-review.googlesource.com/804240Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Luis Hector Chavez <lhchavez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521086}

components/arc/test/connection_holder_util.h

@@ -21,8 +21,8 @@ template <typename InstanceType, typename HostType>
 class ReadinessObserver
     : public ConnectionHolder<InstanceType, HostType>::Observer {
  public:
-  explicit ReadinessObserver(ConnectionHolder<InstanceType, HostType>* holder,
-                             base::OnceClosure closure)
+  ReadinessObserver(ConnectionHolder<InstanceType, HostType>* holder,
+                    base::OnceClosure closure)
       : holder_(holder), closure_(std::move(closure)) {
     holder_->AddObserver(this);
   }