[WebUI] Promote settings-subpage-search to cr-search-field.

This CL migrates the search component that implements
CrSearchFieldBehavior into a shared component directory. This is
required for the activity log page under
chrome://extensions/?activity=<extension_id> to reuse the component for
searching through logs without having to create a duplicate component.

https://chromium-review.googlesource.com/c/chromium/src/+/1365031 will
be updated once this CL is submitted.

Bug: 915031
Change-Id: I0da16636757313a5687068630ad2b60310fb7ec5
Reviewed-on: https://chromium-review.googlesource.com/c/1375455
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616579}
18 files changed