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If pinpoint_traces.py is interrupted while html2trace is busy, the next
extraction would take partial results. To make the process robust in
presence of interruptions, create a temporary directory for html2trace,
then atomically rename it to the directory that will be searched on
extraction.

The operation may seem not equivalent to creating a '.tmp.json' and
renaming it, because this is indeed not equivalent. This change fixes
the bug that I missed in previous testing when I forgot to delete the
json_cache, hence this code was not executing. This time json_cache was
deleted before testing.

Bug: 1119406
Change-Id: I7cfb19572a9ffa26827dd113bb92b2253b0a2b2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637838Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844817}