String vector's comparison works wrong

The problem happened because SimpleClassVectorTraits was used as a vector trait for WTF::String class, hence 'memcmp' was used for string vector's comparison, which is apparently wrong.
This CL corrects vector trait for WTF::String and also introduces a unit test to verify that the problem is not happening any more.

Review URL: https://chromiumcodereview.appspot.com/23819029

git-svn-id: svn://svn.chromium.org/blink/trunk@157360 bbb929c8-8fbe-4397-9dbb-9b2b20218538