Add OWNERS file to fast/webgl and fast/canvas-api

This is a follow-up for the previous CL:
https://chromium-review.googlesource.com/c/chromium/src/+/667323

TBR=junov@chromium.org

Bug: 725651
Change-Id: I438a4210e28ff85d051de5103f33e2b81197f2e4
Reviewed-on: https://chromium-review.googlesource.com/670924Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502823}

third_party/WebKit/LayoutTests/fast/canvas-api/OWNERS

+# TEAM: paint-dev@chromium.org
+# COMPONENT: Blink>Canvas

third_party/WebKit/LayoutTests/fast/webgl/OWNERS

+# TEAM: paint-dev@chromium.org
+# COMPONENT: Blink>Canvas