Ignore sdpSemantics:'plan-b' on Fuchsia.

This is the minimal change necessary to not support enabling Plan B from
JavaScript. This makes Fuchsia's behavior match all other platforms.

Removing the command line flag and cleaning up all the code, including
deprecation counters, should be done in a separate CL after this CL has
had time to "stick".

Bug: 1344773, 1302249
Change-Id: I9f17e9369bffca9e1b5d72134115ab379d50c5ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3762497


Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1025632}