Fix initialization of SimpleClassVectorTraits 'isWeak' and 'needsTracing' properties values

The 'isWeak' and 'needsTracing' properties were recently added to VectorTraitsBase
structure, and values of these properties were not obtained from 'isPod' template
argument (unlike the rest of VectorTraitsBase properties) but rather from the type
template argument 'T'. This however was not taken into consideration in
SimpleClassVectorTraits structure which was inherited VectorTraitsBase in the
following way:

struct SimpleClassVectorTraits : VectorTraitsBase<false, int>

which means that for any trait object inherited from SimpleClassVectorTraits
'isWeak' and 'needsTracing' properties values were calculated using 'int' type
instead of the actual type used.

Now all the VectorTraitsBase properties are initialized from 'T' argument and
the redundant 'isPod' argument is removed. SimpleClassVectorTraits also takes
template type argument 'T' in order to pass it to VectorTraitsBase, so that all
the inherited properties are initialized properly.

Review URL: https://codereview.chromium.org/148083006

git-svn-id: svn://svn.chromium.org/blink/trunk@166109 bbb929c8-8fbe-4397-9dbb-9b2b20218538