Add stevenjb@ to ui/webui/PLATFORM_OWNERS

Bug: none
Change-Id: I23f0200f68aae38baeccb266b20fffe604128651
Reviewed-on: https://chromium-review.googlesource.com/965622Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543752}

ui/webui/PLATFORM_OWNERS

@@ -5,5 +5,6 @@ dpapad@chromium.org
 dschuyler@chromium.org
 michaelpg@chromium.org
 pam@chromium.org
+stevenjb@chromium.org
 tommycli@chromium.org
 xiyuan@chromium.org