Fix TRACE_EVENT snapshotting for LayerImpl

There are two problems with how LayerImpl was instrumented:
1. We never issued TRACE_EVENT_OBJECT_DELETED commands. This meant
   that if a pointer changed from a layer to something else, tracing
   woudl get confused and you would get import errors.
2. We didn't set the category on the snapshot. This meant that
   if a layer was AsValued() into a containing trace of some other category,
   this would cause it in some corner cases to become untraced.

I'm pretty amazed that this ever worked.

R=danakj,enne

Review URL: https://codereview.chromium.org/167893004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251571 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251571