Allow WTF::Vector use memset for POD elements initialization

Set 'canInitializeWithMemset' flag in the corresponding vector trait.

This CL does not make any functional difference at the moment because STD::Vector implementation is checking 'needsInitialization' flag first.

Review URL: https://chromiumcodereview.appspot.com/23819004

git-svn-id: svn://svn.chromium.org/blink/trunk@157079 bbb929c8-8fbe-4397-9dbb-9b2b20218538