Start from clean state in DownloadTests

chrome/android/javatests/src/org/chromium/chrome/browser/download/DownloadTestBase.java

Index: chrome/android/javatests/src/org/chromium/chrome/browser/download/DownloadTestBase.java
diff --git a/chrome/android/javatests/src/org/chromium/chrome/browser/download/DownloadTestBase.java b/chrome/android/javatests/src/org/chromium/chrome/browser/download/DownloadTestBase.java
index e6b33414e0080ad56732ede04702095dbc8c0831..66e8691812e344cc45a2df0a0df450e4840982e3 100644
--- a/chrome/android/javatests/src/org/chromium/chrome/browser/download/DownloadTestBase.java
+++ b/chrome/android/javatests/src/org/chromium/chrome/browser/download/DownloadTestBase.java
@@ -269,4 +269,14 @@ public abstract class DownloadTestBase extends ChromeActivityTestCaseBase<Chrome
         });
         super.tearDown();
     }
+
+    protected void deleteFilesInDownloadDirectory(String...filenames) {
+        for (String filename : filenames) {
+            final File fileToDelete = new File(DOWNLOAD_DIRECTORY, filename);
+            if (fileToDelete.exists()) {
+                assertTrue("Could not delete file that would block this test",
+                        fileToDelete.delete());
+            }
+        }
+    }
 }