Add new is_castos definition to prepare for ANGLE roll (#626)
Add new is_castos definition to prepare for ANGLE roll (#626)

ANGLE renamed `is_chromecast` to `is_castos`: https://chromium-review.googlesource.com/c/angle/angle/+/3782573

This adds the new definition so that we can prepare for the ANGLE roll. Once the roll is complete, we should consider removing the `is_chromecast` definition.

Part of: https://github.com/flutter/flutter/issues/110948
flutter/buildroot,flutter/buildroot,flutter/buildroot,flutter/buildroot