Bug
1882542 - Eagerly Calculate an Initial Translation Download Size
This bug adds:
* When `TranslateExpectedAction` occurs, then calculate the expected
translations model download size.
More background: `TranslateExpectedAction` indicates we have a default
`from` (page language) and a default `to` (user preferred) language
to initialize the translations dialog with expected values.
This queries how large of a download the translation
pair will be eagerly, so we have an initial value for the default recommendation.