[csspaint] Add xidachen, flackr to OWNERS.

Add xidachen@ and flackr@ to modules/csspaint OWNERS.

Change-Id: Ic7da1068210941673604d39351633ab4888bed52
Reviewed-on: https://chromium-review.googlesource.com/600193Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491738}

third_party/WebKit/Source/modules/csspaint/OWNERS

+flackr@chromium.org
 ikilpatrick@chromium.org
 junov@chromium.org
+xidachen@chromium.org
 
 # TEAM: style-dev@chromium.org
 # COMPONENT: Blink>CSS