WebUI TaskManager: Make sort rows including "N/A" correctly. 
WebUI TaskManager: Make sort rows including "N/A" correctly. 

Fixed the problem that sort didn't work correctly when some rows are "N/A". 

BUG=none 
TEST=manually


Review URL: http://codereview.chromium.org/8486014

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@110498 0039d316-1c4b-4281-b951-d872f2087c98

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