[gin] Declare a wrappable's kWrapperInfo on the class deriving from Wrappable

Otherwise, we need to put INIT_WRAPPABLE() in namespace gin no matter
where the derived class is declared.

I pulled this out from https://codereview.chromium.org/111083005/

BUG=none
R=dcarney@chromium.org
TBR=aa@chromium.org

Review URL: https://codereview.chromium.org/113163007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240271 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240271