Simplify checking whether loading is complete

Make DocumentLoader::isLoadingInApiSense a static private in FrameLoader, remove a silly use of it in ResourceFetcher.

Make checkLoadCompleteForThisFrame check its subframes directly in a postorder traversal. This makes FrameLoader::subframeIsLoading() unnecessary and allows us to cleanly ensure we're not checking a frame's state multiple times during the traversal.

BUG=none

Review URL: https://codereview.chromium.org/195023003

git-svn-id: svn://svn.chromium.org/blink/trunk@169061 bbb929c8-8fbe-4397-9dbb-9b2b20218538