[windows] Add a pair of trivial ValueIterator tests.

There was a fear that the value iterator was flaky on Win 7. If that's
true, a little test coverage should expose it.

BUG=1066801
R=robliao@chromium.org

Change-Id: I984044243886510c9c540bc471b6a62a26212e3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2540447
Commit-Queue: Greg Thompson <grt@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828395}

base/win/registry_unittest.cc

@@ -158,6 +158,30 @@ TEST_F(RegistryTest, TruncatedCharTest) {
   EXPECT_FALSE(iterator.Valid());
 }
 
+// Tests that the value iterator is okay with an empty key.
+TEST_F(RegistryTest, ValueIteratorEmptyKey) {
+  RegistryValueIterator iterator(HKEY_CURRENT_USER, root_key().c_str());
+  EXPECT_EQ(iterator.ValueCount(), 0U);
+  EXPECT_FALSE(iterator.Valid());
+}
+
+// Tests that the default value is seen by a value iterator.
+TEST_F(RegistryTest, ValueIteratorDefaultValue) {
+  const WStringPiece kTestString(L"i miss you");
+  ASSERT_EQ(RegKey(HKEY_CURRENT_USER, root_key().c_str(), KEY_SET_VALUE)
+                .WriteValue(nullptr, kTestString.data()),
+            ERROR_SUCCESS);
+  RegistryValueIterator iterator(HKEY_CURRENT_USER, root_key().c_str());
+  EXPECT_EQ(iterator.ValueCount(), 1U);
+  ASSERT_TRUE(iterator.Valid());
+  EXPECT_EQ(WStringPiece(iterator.Name()), WStringPiece());
+  EXPECT_EQ(iterator.ValueSize(), (kTestString.size() + 1) * sizeof(wchar_t));
+  EXPECT_EQ(iterator.Type(), REG_SZ);
+  EXPECT_EQ(WStringPiece(iterator.Value()), kTestString);
+  ++iterator;
+  EXPECT_FALSE(iterator.Valid());
+}
+
 TEST_F(RegistryTest, RecursiveDelete) {
   RegKey key;
   // Create root_key()