GitLab

* Adds the JS side of the safety check browser proxy.
* Introduces the same API on JS side as used on the C++ side
  (https://crrev.com/c/2049982).
* Adds states for the safety check children elements in the form of
  Polymer properties, which will be used by the safety check children
  elements in the future.

Bug: 1015841
Change-Id: I4bd95e2970a018b0d3c8e63114582c1f4e2cb60b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2059375
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744217}