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/threading/thread_checker_unittest.cc b/base/threading/thread_checker_unittest.cc
index 026cfd9e..ae96923 100644
--- a/base/threading/thread_checker_unittest.cc
+++ b/base/threading/thread_checker_unittest.cc
@@ -47,7 +47,7 @@
 // Calls ThreadCheckerClass::DoStuff on another thread.
 class CallDoStuffOnThread : public base::SimpleThread {
  public:
-  CallDoStuffOnThread(ThreadCheckerClass* thread_checker_class)
+  explicit CallDoStuffOnThread(ThreadCheckerClass* thread_checker_class)
       : SimpleThread("call_do_stuff_on_thread"),
         thread_checker_class_(thread_checker_class) {
   }
@@ -65,7 +65,8 @@
 // Deletes ThreadCheckerClass on a different thread.
 class DeleteThreadCheckerClassOnThread : public base::SimpleThread {
  public:
-  DeleteThreadCheckerClassOnThread(ThreadCheckerClass* thread_checker_class)
+  explicit DeleteThreadCheckerClassOnThread(
+      ThreadCheckerClass* thread_checker_class)
       : SimpleThread("delete_thread_checker_class_on_thread"),
         thread_checker_class_(thread_checker_class) {
   }