Use merged JSTest config for performance tests.

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}