Add null-check for likely cause of a top crasher in HistoryController
Add null-check for likely cause of a top crasher in HistoryController

It appears that if we load a javascript into a newly created iframe, we can
create the new frame without creating a new history item (neither real nor
dummy). In that case, HistoryController::restoreDocumentState may crash,
because it assumes m_currentItem is non-null.

BUG=301840
TEST=Haven't reduced one yet, will land one in a followup patch ASAP
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25384002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@158550 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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