If the first load in a Page is a history load, ensure we use the main frame

Currently, it is possible that we will get confused between "The desired frame
doesn't exist in the current history entry" and "There is no current history
entry because we are in a new Page". In the later case, if the history load
includes an about:blank child item, we might accidentally load that into the
main frame instead.

BUG=332377
TEST=WebFrameTest.firstNavigationIsHistoryWithBlankChild

Review URL: https://codereview.chromium.org/139993002

git-svn-id: svn://svn.chromium.org/blink/trunk@165201 bbb929c8-8fbe-4397-9dbb-9b2b20218538