Add test for OfflinePageRequestHandler validating an empty archive file

This fixes a bug causing OfflinePageRequestHandler to segfault when validating
empty archive files and adds a test to ensure empty archives can be validated.

Bug: 913608
Change-Id: I043cbb0c778f9244763197f294c3839f854c71f2
Reviewed-on: https://chromium-review.googlesource.com/c/1370715Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Dan H <harringtond@google.com>
Commit-Queue: Ian Wells <iwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615618}

chrome/browser/offline_pages/offline_page_request_handler.cc

@@ -797,7 +797,8 @@ void OfflinePageRequestHandler::UpdateDigestOnBackground(
 
 void OfflinePageRequestHandler::FinalizeDigestOnBackground(
     base::OnceCallback<void(const std::string&)> digest_finalized_callback) {
-  DCHECK(archive_validator_.get());
+  if (!archive_validator_)
+    archive_validator_ = new ThreadSafeArchiveValidator();
 
   // Delegate to background task runner to finalize the hash to get the digest
   // since it is time consuming. Once it is done, |digest_finalized_callback|

chrome/browser/offline_pages/offline_page_request_handler_unittest.cc

@@ -2077,6 +2077,26 @@ TYPED_TEST(OfflinePageRequestHandlerTest, LoadOtherPageOnDigestMismatch) {
   this->ExpectOfflinePageAccessCount(offline_id2, 0);
 }
 
+TYPED_TEST(OfflinePageRequestHandlerTest, EmptyFile) {
+  this->SimulateHasNetworkConnectivity(false);
+
+  const std::string expected_data("");
+  base::FilePath temp_file_path = this->CreateFileWithContent(expected_data);
+  ArchiveValidator archive_validator;
+  const std::string expected_digest = archive_validator.Finish();
+
+  int64_t offline_id =
+      this->SavePublicPage(kUrl, GURL(), temp_file_path, 0, expected_digest);
+
+  this->LoadPage(kUrl);
+
+  this->ExpectOfflinePageServed(
+      offline_id, 0,
+      OfflinePageRequestHandler::AggregatedRequestResult::
+          SHOW_OFFLINE_ON_DISCONNECTED_NETWORK);
+  EXPECT_EQ(expected_data, this->data_received());
+}
+
 TYPED_TEST(OfflinePageRequestHandlerTest, TinyFile) {
   this->SimulateHasNetworkConnectivity(false);