Add missing, and remove unnecessary, 'explicit' from constructors. (base/)

The style guide says that constructors which can be called with one
argument should be explicit.

For constructors which cannot be called with exactly one argument,
there is no reason to mark them explicit.

BUG=163357


Review URL: https://chromiumcodereview.appspot.com/11779022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176881 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/task_runner.cc b/base/task_runner.cc
index b0563d4..5860f28 100644
--- a/base/task_runner.cc
+++ b/base/task_runner.cc
@@ -17,7 +17,7 @@
 // possible to merge the two.
 class PostTaskAndReplyTaskRunner : public internal::PostTaskAndReplyImpl {
  public:
-  PostTaskAndReplyTaskRunner(TaskRunner* destination);
+  explicit PostTaskAndReplyTaskRunner(TaskRunner* destination);
 
  private:
   virtual bool PostTask(const tracked_objects::Location& from_here,