Add missing (and remove superfluous) 'explicit' from constructors.

base/threading/worker_pool.cc

Index: base/threading/worker_pool.cc
diff --git a/base/threading/worker_pool.cc b/base/threading/worker_pool.cc
index 16cc061648df6e5d9cf8e74c20bb460b6c055a48..9e45f8c89b8ea7964e73bfb328ffc3e2e3694407 100644
--- a/base/threading/worker_pool.cc
+++ b/base/threading/worker_pool.cc
@@ -17,7 +17,8 @@ namespace {
 
 class PostTaskAndReplyWorkerPool : public internal::PostTaskAndReplyImpl {
  public:
-  PostTaskAndReplyWorkerPool(bool task_is_slow) : task_is_slow_(task_is_slow) {
+  explicit PostTaskAndReplyWorkerPool(bool task_is_slow)
+      : task_is_slow_(task_is_slow) {
   }
 
  private:
@@ -36,7 +37,7 @@ class PostTaskAndReplyWorkerPool : public internal::PostTaskAndReplyImpl {
 // Note that this class is RefCountedThreadSafe (inherited from TaskRunner).
 class WorkerPoolTaskRunner : public TaskRunner {
  public:
-  WorkerPoolTaskRunner(bool tasks_are_slow);
+  explicit WorkerPoolTaskRunner(bool tasks_are_slow);
 
   // TaskRunner implementation
   virtual bool PostDelayedTask(const tracked_objects::Location& from_here,