The FLX will contain a font manifest JSON file that maps font family names
to custom font assets. Flutter will provide a FontSelector that loads
fonts on demand and caches typeface and style data.
Currently, during application startup we:
* copy the FLX file to a Mojo pipe
* write the Mojo pipe contents back to a temporary file on disk
* unzip the FLX contents into cache storage
This contributes to startup latency and requires that we later clean up the
cache.
With this change, the assets will be extracted from the FLX archive
on demand with no writes to storage. Runtime cost should be minimal
given that most assets (except for the snapshot) are not compressed
in the archive.
This updates to mojo commit d259eb58aa59 and limits the roll script to
only pull in the parts of //mojo that are currently being used. More
stuff will be dropped in the future.
