[owners] Add thibaudm and ecmziegler to COMMON_OWNERS

There already became committers some month ago.

R=adamk@chromium.org
CC=​ecmziegler@chromium.org, thibaudm@chromium.org

Change-Id: Icac1c0265f4928dae3b89105cbc6f843f1d4e2af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2210243Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67932}

COMMON_OWNERS

@@ -9,6 +9,7 @@ clemensb@chromium.org
 danno@chromium.org
 delphick@chromium.org
 dinfuehr@chromium.org
+ecmziegler@chromium.org
 gdeepti@chromium.org
 gsathya@chromium.org
 hablich@chromium.org
@@ -34,6 +35,7 @@ solanes@chromium.org
 syg@chromium.org
 szuend@chromium.org
 tebbi@chromium.org
+thibaudm@chromium.org
 ulan@chromium.org
 verwaest@chromium.org
 victorgomes@chromium.org