Large parallelism causes too many open files error.
Let me limit to 500 for Mac now.

Bug: 936864
Change-Id: I2fccc8cf14483c6f34d84c84d82c44df6e4f3177
Reviewed-on: https://chromium-review.googlesource.com/c/1496675Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>