Remove warnings in org.chromium.chrome.browser.ntp.* code and JUnit tests.

chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
index cb844104663a4cc6eed0ded980583984f27484af..6bcd4d41dde35693146262ae324eacb0f205da68 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
@@ -134,20 +134,22 @@ public class SnippetArticle {
     }
 
     /**
-     * @return the asset download path. May only be called if {@link mIsAssetDownload} is
-     * {@code true} and this snippet belongs to DOWNLOADS category.
+     * @return the asset download path. May only be called if {@link #mIsAssetDownload} is
+     * {@code true} (which implies that this snippet belongs to the DOWNLOADS category).
      */
     public File getAssetDownloadFile() {
         assert isDownload();
+        assert mIsAssetDownload;
         return mAssetDownloadFile;
     }
 
     /**
-     * @return the mime type of the asset download. May only be called if {@link mIsAssetDownload}
-     * is {@code true} and this snippet belongs to DOWNLOADS category.
+     * @return the mime type of the asset download. May only be called if {@link #mIsAssetDownload}
+     * is {@code true} (which implies that this snippet belongs to the DOWNLOADS category).
      */
     public String getAssetDownloadMimeType() {
         assert isDownload();
+        assert mIsAssetDownload;
         return mAssetDownloadMimeType;
     }