WebUI Polymer2: Remove Polymer1 support, part 2.

Removing kWebUIPolymer2 flag. It has been turned on by default
on all platforms, since M72 for Linux/Mac/Win, and M74 for CrOS.

Also removing
 - ShadowDOMV0, CustomElementsV0 exceptions for PDF viewer
 - ShadowDOMV0, CustomElementsV0, HTMLImports exceptions for File Manager

since those should no longer be needed.

Bug: 944281
Change-Id: I7f684c4102cfc0a1a88149a16c30acce7e50cca7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532892
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653045}
14 files changed