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This is the last part of a three-sided change in which the return value
of StopTracing() was changed from a TraceValue object to a (TraceValue
object, list of nonfatal exceptions) tuple.

Bug: catapult:#3798
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I6e237ce13f7cf263ca453fa47c859f65d1c051b8
Reviewed-on: https://chromium-review.googlesource.com/827549Reviewed-by: Charlie Andrews <charliea@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524162}