Deflake cache-storage/script-tests/cache-storage-match.js WPT

The test assumed no cache named 'foo' existed, but another test
cache-keys-attributes-for-service-worker creates a cache by that name,
so the test would flakily fail in some runs depending on the ordering
of the tests.

Make the test defensive by deleting such a cache before proceeding.

Bug: 840850
Change-Id: Ia349706c9a32af309cfadf9a63ba5cffea11998d
Reviewed-on: https://chromium-review.googlesource.com/1119104
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572232}

third_party/WebKit/LayoutTests/external/wpt/service-workers/cache-storage/script-tests/cache-storage-match.js

@@ -117,7 +117,10 @@ promise_test(function(test) {
 
 promise_test(function(test) {
     var transaction = create_unique_transaction();
-    return self.caches.has('foo')
+    return self.caches.delete('foo')
+      .then(function() {
+          return self.caches.has('foo');
+        })
       .then(function(has_foo) {
           assert_false(has_foo, "The cache should not exist.");
           return self.caches.match(transaction.request, {cacheName: 'foo'});