Keep track of SDP semantics, not including kDefault.

SDP semantics can be passed in to the PC as kPlanB, kUnifiedPlan or
kDefault.

This CL does the translation from kDefault to kPlanB (default) or
kUnifiedPlan (if a flag is used to overwrite the default) at
construction once and for all. This eliminates assumptions about what
kDefault may mean in different parts of the code or layers.

RTCPeerConnectionHandler is also updated to keep track of the SDP
semantics. Also passes down "sdp_semantics_requested" for the sake of
UMA counters which is the exception where we do care if "kDefault" was
used or not by the application.

This CL also updates some Plan B-only methods to DCHECK SDP semantics
or renames them to "...PlanB()".

// Bots have already been successful, skipping remaining bot which is
// inactive.
NOTRY=True

Bug: 777617
Change-Id: If88017360fa577297796bac4faf4e7bc56bd99d0
Reviewed-on: https://chromium-review.googlesource.com/1141880
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576171}
14 files changed