Merge FrameLoader::stopLoading into stopAllLoaders

FrameLoader::stopLoading is called twice:
* From FrameLoader::closeURL after the unload event, when the relevant document is about to disappear and is going to get cleaned up anyway.
* From DocumentLoader::stopLoading in the case where the document is committed. The only time this occurs is within FrameLoader::stopAllLoaders.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23361010

git-svn-id: svn://svn.chromium.org/blink/trunk@156582 bbb929c8-8fbe-4397-9dbb-9b2b20218538