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This change adds the chooser-exception-list Polymer element to site
settings. This element will fetch the chooser exceptions from the C++
site and process them for display. The element uses a
chooser-exception-list-entry to render each chooser exception in the
list.

This change also adds to ability for the paper-tooltip to have its
target set manually. This allows the target to be set on the common
paper-tooltip element used in chooser-exception-list and site-list
without having to access its private `_target` property.

Design doc:
https://docs.google.com/document/d/1MPvsrWiVD_jAC8ELyk8njFpy6j1thfVU5aWT3TCWE8w

Bug: 854329
Change-Id: I3f127d2cf655b2d75157d7174a9bd5e434b583a8
Reviewed-on: https://chromium-review.googlesource.com/c/1357556
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622059}