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This adds internal slots "LastOffer" and "LastAnswer", and uses
those to check that SetLocalDescription uses an unchanged SDP offer/answer.

Because modification of SDP is suspected to be used in a number of places,
this change only rejects SDP with modified fingerprints (which would fail
anyway), but merely counts the usage for other modifications.

Bug: chromium:823036
Change-Id: I0c978d5ff3e63b0afab3ec02334c57a5aaa94cdd
Reviewed-on: https://chromium-review.googlesource.com/966441
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545175}