[web] Avoid overwriting line-height in ChatInputBar textarea

Summary: This is breaking Safari's ability to automatically adjust line-height when it sees emojis.

Test Plan: Make sure emoji appear correctly in textarea on Safari

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D959