Simplify parsing/loading state, attempt #2

It seemed strange that DocumentLoader::stopLoading() both stops
fetching via ResourceFetcher::stopFetching() and reports a
cancellation via DocumentLoader::cancelMainResourceLoad().

Turns out, a lot of legacy logic is only needed to make that
cancelMainResourceLoad() call work correctly. Replace it with
a smaller, more container hack in FrameLoader::stopAllLoaders.

BUG=

Review URL: https://codereview.chromium.org/1823863002

Cr-Commit-Position: refs/heads/master@{#382892}
14 files changed