Make DownloadController singleton instance thread safe.

As the cost of creating download controller instance is not
too heavy its better to switch to eager initialization which
creates a ready to use instance and ensures thread safety

This method has a number of advantages:
* The instance is not constructed until the class is used.
* There is no need to synchronize the getInstance() method,
  meaning all threads will see the same instance and no (expensive)
  locking is required.
* The final keyword means that the instance cannot be redefined,
  ensuring that one (and only one) instance ever exists.
(source: http://en.wikipedia.org/wiki/Singleton_pattern#Eager_initialization)

BUG=None.

Review URL: https://codereview.chromium.org/387333002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283430 0039d316-1c4b-4281-b951-d872f2087c98

content/public/android/java/src/org/chromium/content/browser/DownloadController.java

@@ -17,7 +17,7 @@ import org.chromium.base.JNINamespace;
 @JNINamespace("content")
 public class DownloadController {
     private static final String LOGTAG = "DownloadController";
-    private static DownloadController sInstance;
+    private static final DownloadController sInstance = new DownloadController();
 
     /**
      * Class for notifying the application that download has completed.
@@ -40,9 +40,6 @@ public class DownloadController {
 
     @CalledByNative
     public static DownloadController getInstance() {
-        if (sInstance == null) {
-            sInstance = new DownloadController();
-        }
         return sInstance;
     }