Buffer11: Use adaptive threshold for releasing system memory. The hard-coded threshold of five uses before a release was regressing the Oort online benchmark, which seems to use an index buffer many times, then change to a different range of indices, which would need to check the system memory for index range validation. Also add a performance regression test, and an update to the perf runner script which checks for the most recent binary among the search directories. BUG=594066 Change-Id: Id09cc32fd00bff1c72cbe9b6fb7c210fd047a551 Reviewed-on: https://chromium-review.googlesource.com/339271 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>