Simplification WTF::VectorTraitsBase class

WTF::VectorTraitsBase class does not actually need specifications, as its template parameter 'isPod' can be used to define the flags values.
Furthermore assigning flags to 'true' or 'false' directly is error prone (pls. see https://codereview.chromium.org/23819004/).

Review URL: https://chromiumcodereview.appspot.com/23533022

git-svn-id: svn://svn.chromium.org/blink/trunk@157162 bbb929c8-8fbe-4397-9dbb-9b2b20218538