Upcoming changes to Polymer Resin will make it easier to mark values as
safe without using closure libraries, but will also make binding `href`
attributes of links more strict, effectively requiring the use of
Closure unless an alternative safe-types implementation is configured.
Add a safe-types implementation as a Polymer behavior. This permits some
strings to be used in `href` bindings as is. It also adds a SafeUrl type
that Polymer elements can use to explicitly mark URLs that will be used
for `href`s.
The safe types implementation also provides a `safeTypesBridge` mapping
that is configured into Polymer Resin. This mapping permits safe URLs
and regular strings that are typical, but it is more restrictive than
vanilla Resin by totally rejecting other types of binding that PG does
not use, such as resource-URL bindings or bindings for raw HTML.
Change-Id: I91c4fe041eed286a17f01d5c1b12e215b15378b0
bef3d20ba9