GitLab

 - Adding a new c/b/r/settings/settings.gni file.
 - Defining |settings_namespace_rewrites| and |settings_auto_imports|
   holding common Settings-related entries
 - Also extract a few non-Settings common rewrites to
   tools/polymer/polymer.gni

This is so that they can be reused across multiple BUILD.fn files.

Bug: 1026426
Change-Id: I11ee3b4e30d3e4871b9483e2a7a9ff5ec8bae7fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032568
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737426}