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Like the recent rename of MediaSourceAttachmentImpl to
SameThreadMediaSourceAttachment [1], this change renames
MediaSourceTracerImpl to SameThreadMediaSourceTracer. There is no
behavior change.

This change makes it more clear that the tracer being renamed here is
the same-thread version, since later changes will introduce a
cross-thread version.

[1] https://crrev.com/803635

BUG=878133

Change-Id: I3cfa000db21ca87fe79bf73ea2d2490c1976df32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390966Reviewed-by: Will Cassella <cassew@google.com>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804446}