1. Remove estade@ from main OWNERS file
   👋 So long and thanks for all the fish! 🐟🐠

2. Create a "PLATFORM_OWNERS" file so that fukino@ can be added
   for only ui/webui/ (and not all the other things covered).

3. Add calamity@ to new PLATFORM_OWNERS file.

4. Add groby@ to content/browser/webui/ for her super contributions.

5. Add dschuyler@ to ui/base/webui/ for his sweet $i18n{} stuff.

R=estade@chromium.org,fukino@chromium.org,calamity@chromium.org
BUG=none
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2812803002
Cr-Commit-Position: refs/heads/master@{#465902}