[ffmpeg] roll ffmpeg to 346d30b3f6631f2d47fa310f7dfeb78215706fc3

This CL rolls ffmpeg forward to use a current version of chromium's
third_party/ffmpeg repo. This is the first roll using the new roll
scripts and the fuchsia main GN files that specify what features to
include.

For the time being, we still have the 'default' and 'max' profiles
that have existing for some time. 'max' has been the profile used
by default for both prebuilds and use_ffmpeg_prebuilt=false builds.

We want the 'default' profile to be the default profile going
forward, and for 'max' to go away. We will also introduce product-
specific profiles. In order to start using 'default' as the default
profile, the 'default' profile has been updated to be identical to
the 'max' profile, so that transition can occur without disruption.
Both profiles differ in benign ways from the previous 'max' profile.

In addition to the roll, this CL makes the following changes:

1) Fixed an error in the roll instructions.
2) Updated to the new lib version numbers.
3) Made the non-prebuild build use 'default' by default.

Change-Id: I215e38c41b26ea96fb91586ba7b510867774d7d2
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/ffmpeg/+/807596
Reviewed-by: Petr Hosek <phosek@google.com>
33 files changed