Fix an issue that download entries in SharedPreferences is not removed when exiting app

SharedPreference.edit() creates a new editor.
So calling this method twice creates 2 different editors.
And the apply() call from the 2nd editor has no effect on the first editor.

BUG=459403

Review URL: https://codereview.chromium.org/930303003

Cr-Commit-Position: refs/heads/master@{#316956}

chrome/android/java/src/org/chromium/chrome/browser/download/DownloadManagerService.java

@@ -200,8 +200,7 @@ public class DownloadManagerService extends BroadcastReceiver implements
                     Log.w(TAG, "Download failed: Cleared download id:" + id);
                 }
             }
-            mSharedPrefs.edit().remove(DOWNLOAD_NOTIFICATION_IDS);
-            mSharedPrefs.edit().apply();
+            mSharedPrefs.edit().remove(DOWNLOAD_NOTIFICATION_IDS).apply();
         }
         if (mSharedPrefs.contains(PENDING_OMA_DOWNLOADS)) {
             Set<String> omaDownloads = getStoredDownloadInfo(PENDING_OMA_DOWNLOADS);