expansionOpportunityCount should be 'unsigned' rather than 'int'

Since the return type of expansionOpportunityCount function is 'unsigned',
expansionOpportunityCount variable should be treated as 'unsigned'.

Review URL: https://codereview.chromium.org/254803009

git-svn-id: svn://svn.chromium.org/blink/trunk@172780 bbb929c8-8fbe-4397-9dbb-9b2b20218538