Make manifest section of bundled exchanges optional

Spec side PR: https://github.com/WICG/webpackage/pull/505

TBR=palmer@chromium.org

Bug: 969596
Change-Id: I862fd24c2b3d8a4b237d0852829361dcf9cdde40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1861614Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706749}

services/data_decoder/bundled_exchanges_parser.cc

@@ -596,16 +596,12 @@ class BundledExchangesParser::MetadataParser
     // Step 23. "Assert: metadata has an entry with the key "primaryUrl"."
     DCHECK(!metadata_->primary_url.is_empty());
 
-    // Step 24. "If metadata doesn't have entries with keys "requests" and
-    // "manifest", return a "format error" with fallbackUrl."
+    // Step 24. "If metadata doesn't have an entry with the key "requests",
+    // return a "format error" with fallbackUrl."
     if (metadata_->requests.empty()) {
       RunErrorCallbackAndDestroy("Bundle must have an index section.");
       return;
     }
-    if (metadata_->manifest_url.is_empty()) {
-      RunErrorCallbackAndDestroy("Bundle must have a manifest URL.");
-      return;
-    }
 
     // Step 25. "Return metadata."
     RunSuccessCallbackAndDestroy();

services/data_decoder/bundled_exchanges_parser_unittest.cc

@@ -635,7 +635,8 @@ TEST_F(BundledExchangeParserTest, NoManifest) {
                       "payload");
   TestDataSource data_source(builder.CreateBundle());
 
-  ExpectFormatErrorWithFallbackURL(ParseBundle(&data_source));
+  mojom::BundleMetadataPtr metadata = ParseBundle(&data_source).first;
+  ASSERT_TRUE(metadata);
 }
 
 TEST_F(BundledExchangeParserTest, InvalidManifestURL) {