Fix isac_fix_test perf reporting.
Fix isac_fix_test perf reporting.

The regression has been introduced by [1] and here is an example of
the problem [2].

[1] - https://webrtc.googlesource.com/src/+/2ab97f6f8e27b47c0d9beeb8b6ca5387bda9f55c
[2] - https://ci.chromium.org/p/webrtc/builders/perf/Perf%20Linux%20Trusty/1543

TBR=minyue@webrtc.org

Bug: webrtc:10616
Change-Id: I109b55a612d49981ce74c3e4cd50603ee0003250
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146540
Commit-Queue: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Reviewed-by: Oleh Prypin <5baa3e04f9bd6ba7a2a4b56625a3c84f19237cdb@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#28627}

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