Reduce usage of FrameLoader::m_isComplete.

After this patch, it is only used internally to determine checkCompleted()
behavior. It is almost-but-not-quite synonymous with
Document::loadEventFinished().

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22999017

git-svn-id: svn://svn.chromium.org/blink/trunk@156180 bbb929c8-8fbe-4397-9dbb-9b2b20218538