Fix PopupListBox::m_selectedIndex from going out of bounds after r189396
Fix PopupListBox::m_selectedIndex from going out of bounds after r189396

When the dom is modified and the size of m_items becomes smaller, m_selectedIndex can become out of bounds.

BUG=456881

Review URL: https://codereview.chromium.org/921653002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190112 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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