Use std::fstream instead of rtc::File to write perf results.
On Android, when I use rtc::File, the results are not written for some reason.
Also rename the flag to '--chartjson_result_file'.
Bug: webrtc:8566
Change-Id: I32215e2233e18690c41050dfd35ac77e01d11f35
Reviewed-on: https://webrtc-review.googlesource.com/32001
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21225}
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