Fix uninitialized pointer in unit tests.

chrome/browser/android/offline_pages/prerendering_offliner_unittest.cc

Index: chrome/browser/android/offline_pages/prerendering_offliner_unittest.cc
diff --git a/chrome/browser/android/offline_pages/prerendering_offliner_unittest.cc b/chrome/browser/android/offline_pages/prerendering_offliner_unittest.cc
index 7e73c92ed4fc1d5f45adf27354c769ae9606c95c..cc95fccc16b69f6cfbcc39657cd1977aa6a82638 100644
--- a/chrome/browser/android/offline_pages/prerendering_offliner_unittest.cc
+++ b/chrome/browser/android/offline_pages/prerendering_offliner_unittest.cc
@@ -45,7 +45,8 @@ class MockPrerenderingLoader : public PrerenderingLoader {
         mock_loading_(false),
         mock_loaded_(false),
         mock_is_lowbar_met_(false),
-        start_snapshot_called_(false) {}
+        start_snapshot_called_(false),
+        web_contents_(nullptr) {}
   ~MockPrerenderingLoader() override { delete web_contents_; }
 
   bool LoadPage(const GURL& url,