Add a RefPtr<Frame> for child in FrameLoader::checkLoadCompleteForThisFrame

Not having this can cause problems in the test harnass, where some of the
FrameLoaderClients do substantially more work than they do in a non-test
environment.

BUG=357283

Review URL: https://codereview.chromium.org/317513002

git-svn-id: svn://svn.chromium.org/blink/trunk@175601 bbb929c8-8fbe-4397-9dbb-9b2b20218538