[gin] WrappableBase's dtor needs to be virtual

Otherwise, the weak callback won't run the dtor of derived classes.

Also clarify that the INIT_WRAPPABLE() macro needs to be called in
namespace gin.

BUG=none
R=aa@chromium.org

Review URL: https://codereview.chromium.org/93903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240154 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240154