Update various WebUI related OWNER files.

Bug: None
Change-Id: I3fb8f1b55f913960669d69bdf3e301efd8f1bd02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057640
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742450}

chrome/browser/resources/settings/OWNERS

+aee@chromium.org
 dpapad@chromium.org
 dschuyler@chromium.org
-hcarmona@chromium.org
 khorimoto@chromium.org
 michaelpg@chromium.org
 tommycli@chromium.org

chrome/browser/resources/welcome/OWNERS

-hcarmona@chromium.org
+johntlee@chromium.org

chrome/browser/ui/webui/welcome/OWNERS

-hcarmona@chromium.org
+johntlee@chromium.org
 
 # COMPONENT: UI>Browser>FirstRun

third_party/node/OWNERS

 dpapad@chromium.org
+rbpotter@chromium.org
 
 # COMPONENT: Build

third_party/polymer/OWNERS

 dpapad@chromium.org
 michaelpg@chromium.org
+rbpotter@chromium.org
 
 # COMPONENT: UI>Browser>WebUI

tools/polymer/OWNERS

 dpapad@chromium.org
 michaelpg@chromium.org
+rbpotter@chromium.org

ui/webui/PLATFORM_OWNERS

@@ -3,7 +3,6 @@ aee@chromium.org
 calamity@chromium.org
 dpapad@chromium.org
 dschuyler@chromium.org
-hcarmona@chromium.org
 michaelpg@chromium.org
 rbpotter@chromium.org
 tommycli@chromium.org