Make WTF::IsPointerConvertible and WTF::InstantiateOnlyWhen more consistent with other WTF type traits.

WTF::IsPointerConvertible implementation was modified to be consistent with WTF::IsConvertibleToInteger, WTF::IsSubclass and similar type traits implemenations.
The COMPILE_ASSERT's verifying WTF::IsPointerConvertible behavior were added into TypeTraits.cpp.

WTF::InstantiateOnlyWhen was renamed to WTF::EnableIf to be more consistent with STL (as well as other WTF type traits do) its implemenation was modified accordingly.

Review URL: https://chromiumcodereview.appspot.com/23449006

git-svn-id: svn://svn.chromium.org/blink/trunk@156856 bbb929c8-8fbe-4397-9dbb-9b2b20218538