Use merged JSTest config for performance tests.

The new config is now used for perf measurements. The old
ones are deleted to avoid redundancy.

This also ports https://codereview.chromium.org/754523004 to the new config file.

BUG=chromium:374740
LOG=n
TBR=ulan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/782673003

Cr-Commit-Position: refs/heads/master@{#25674}