I'll try to explain the idea behind this change, sometimes it's hard to explain with text only when talking about markup!

Currently we have a section `thread-messages` which contains several and well defined regions: header, subheader... and a set of elements which use the rest of the space available, as `information-participants` or `information-report`. Check:
https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/index.html#L209
https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/index.html#L212

However, and due to v1.0 (it seems that was decades ago!), `messages-container` and `messages-subject-input` were not sharing the same *space*, why? Because in the first version of SMS App, the input was growing without taking into account the list of messages.

Then, UX decided to make them collaborate sharing the available space (remember, the height of `thread-messages` without the one dedicated to the `header` & `subheader`), so we included a lot of JS to do this magic. This was added in v1.3 with MMS as you know.
This patch is trying to remove that logic, so the way of sharing the space is done by a flex layout sharing the available space in a natural way. How? Creating a wrapper at the same level as `information-participants` or `information-report` (the ones which use the available space taking into account header and subheader), and including `messages-container` and `messages-subject-input` within it. Result? They *share* the same height without JS magic, and we are respecting the previous markup.