Move file web_links into the meta_a, meta_b headers

Web links should be part of the metadata for the two halves of a diff.
The protocol uses meta_a and meta_b to represent the halves, with
contained fields for specific values.

Also avoid unnecessary copying of the WebLinkInfo by reusing the
created list from the WebLinks factory.

Change-Id: I2c31cad2a4fed11e513f81cd613742534662a663
6 files changed