Add a missing "const" for constructor parameter.
Add a missing "const" for constructor parameter.

TBR=abarth

Review URL: https://chromiumcodereview.appspot.com/24246017

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@158168 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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