This adds an option `--enable-unstable-features`, which now also turns
on multimodal support for Freestyler.
It also adds `--enable-features` and `--disable-features`, to allow more
fine-grained control over the exact feature set when necessary.
Fixed: 406941932
Change-Id: I7fcecbba833a2e0a66166890aa5af218ae2a8f7e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6404318
Reviewed-by: Nikolay Vitkov <nvitkov@chromium.org>
