[lib] Add explicit type to messageContent functions

Summary: It appears that flow has some issues in determining the type of parameters inside spec functions. This is the first diff which adds explicit type annotations.

Test Plan: Flow

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D758