Fix possible use-after-free in WGC3D shared context map

Remove WGC3D from the map in Destroy() rather than ~WGC3DCBImpl(),
because removal from the multimap needs |host_|.
Otherwise it's possible that MaybeInitializeGL() and CreateContext()
fail and call Destroy(), which resets |host_|, and we already inserted
ourselves in the map.

BUG=325071

Review URL: https://codereview.chromium.org/102823002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238546 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238546