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https://chromium-review.googlesource.com/c/566806 showed that the
WebRtcRtpBrowserTest.AddAndRemoveTracks* tests would reliably crash
if the garbage collector was invoked between addTrack and getSenders.
Tracks that were added using addTrack would have their track adapters
(glue between blink and webrtc) kept alive only by the blink layer
sender holding on to a reference, since these were not kept alive by
any "local streams" holding a reference.

The blink::RTCPeerConnection now holds a strong reference to senders
and receivers to prevent GC while in-use, with TODOs to remove ones
no longer used when addStream/removeStream is implemented using
addTrack/removeTrack.

Furthermore the content::RTCPeerConnectionHandler holds on to the
content layer representation of senders so that their associated set
of streams are not forgotten between addTrack and getSenders.

Having the handler keep track of senders is good practice, this gets
rid of the assumption that blink layer senders have to be kept alive.
A TODO was added to do the same for receivers (not yet required because
blink layer receivers are kept alive).

With this change, the AddAndRemoveTracks* tests pass and are
re-enabled.

Bug: 740650
Change-Id: Iec3a39a994bef31904f2969d791125867ec1e398
Reviewed-on: https://chromium-review.googlesource.com/567184Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Taylor Brandstetter <deadbeef@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487430}