Fix missing isolated output directory.
Fix missing isolated output directory.

This change aims at fixing swarming errors like the below:
https://chrome-swarming.appspot.com/task?d=true&id=4e548270683c1610
Adding directory in the output path is used by Pinpoint:
https://source.chromium.org/chromium/chromium/src/+/master:third_party/catapult/dashboard/dashboard/pinpoint/models/quest/read_value.py;l=77

Bug: webrtc:11084
Change-Id: If725fd9f87961a86d91f04f25fe283c0be4e0e33
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/182920
Commit-Queue: Jeremy Leconte <24583aea4726e922fc48ee725ba1b8aadbe9fd66@webrtc.org>
Reviewed-by: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#32018}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc