[WPT] Add armax to OWNERS file.

From the review in http://crrev.com/c/1584271 it was suggested that I should be
added to the OWNERS file of the WPT tests for mediacapture-streams for
communication purposes. This CL adds the armax@chromium.org address as owner.

BUG=None

Change-Id: Ic2fedd21414ac25e28ffbfbe50c707d045e76433
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1584308Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Armando Miraglia <armax@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654431}

third_party/blink/web_tests/external/wpt/mediacapture-streams/OWNERS

 # COMPONENT: Blink>MediaStream
 # WPT-NOTIFY: true
 guidou@chromium.org
+armax@chromium.org