Settings: Make subpage no-search mechanism work in Polymer 2.

In Polymer 2 the following pattern throws a runtime error:
<template is="dom-if" no-search$="[[foo]]>...</template>

This CL introduces a backward compatible solution such that the no-search
mechanism works both in Polymer 1 and Polymer 2. Some cleanups can be done
once migration has completed.

Bug: 859689
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I0732a8c6cd48b94ad3d81cc017fe59688fe3df13
Reviewed-on: https://chromium-review.googlesource.com/1123711
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573968}
5 files changed