Collect page meta info and serialize it in the head (bug 45206).

Right now we just add the data to this.env.page.meta.  A later patch will
serialize some of this in the <head> for users of the external API.

Change-Id: Ib7b866b899cfbcf47d8a06190a411d1a3d46dd50
Merge "Collect page meta info and serialize it in the head (bug 45206)."