WebUI: Import Polymer3 in third_party/polymer.

 - Add new third_party/polymer/v3_0/ folder.
 - Add package.json, package-lock.json and rsync_exclude.txt.
 - Add reproduce.sh script to update the v3 version.
 - Add chromium.patch with some initial local modifications.
 - Update existing scripts polymer_grdp_to_txt.py and
   txt_to_polymer_grdp.py to work for v3 as well.
 - Update shared_resources_data_source.cc to properly map
   chrome://resources/polymer/v3_0/ requests to files.

Note: Given that Polymer 3 usage in WebUI is still at an early
exploration phase, its resources are only included in the build for
optimize_webui=false, to not affect release builds for now, while
allowing making progress on other fronts like
 - type checking
 - bundling
 - HTML-to-JS automation
 - testing JS module based code wit js2gtest infra

Bug: 965770
Test: Visit chrome://resources/polymer/v3_0/paper-button/paper-button.js
Change-Id: I9a6f2c0d9c59d9569bd0f0f4d09ce94a1270cebd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610718
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663383}
162 files changed