Re-enable use of ParagraphConstrains const ctor (#26990)

All embedders of Flutter within Google have all been updated to an
engine version that includes the const constructor added in
flutter/engine#7346 and rolled into the framework in
flutter/flutter#26252.

This reverts commit 88b77a7d0fe5b00baaf63280d48398cf85a99ce6.

This fixes https://github.com/flutter/flutter/issues/26390.