[Chromecast] Change chromecast_branding to public

Change chromecast_branding from Chrome/Chromium to
public/internal/google in gyp file. "public" is the same as "Chromium"
and the other two are considered as "Chrome".

BUG=

Review URL: https://codereview.chromium.org/1262673004

Cr-Commit-Position: refs/heads/master@{#341016}
8 files changed