[wptrunner] Increase parallelism from 3 processes to 20.

Based on my testing[0], this is a reasonable sweet spot before
performance gains diminish substantially (and then actually start
to regress near 40 processes).

Long term, we should consider the user/bot's cpu_count and calculate
something like that, but for now this brings our shard time down from
nearly 300m to ~130m.

[0]: https://docs.google.com/spreadsheets/d/1hMIoOB1oaC9iyWK5GEy5xIwUJLoDRaI77ReJ1lKjpZk/edit#gid=0

Bug: 1023835

Change-Id: Ifccb39af65f8ac9e785818b3e4cf27138dc61d26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633371Reviewed-by: Luke Z <lpz@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844837}

testing/buildbot/chromium.fyi.json

@@ -51664,7 +51664,7 @@
     "isolated_scripts": [
       {
         "args": [
-          "--child-processes=3"
+          "--child-processes=20"
         ],
         "isolate_name": "wpt_tests_isolate",
         "merge": {

testing/buildbot/test_suites.pyl

@@ -5105,7 +5105,7 @@
     'wpt_web_tests': {
       'wpt_tests_suite': {
         'args': [
-          '--child-processes=3',
+          '--child-processes=20',
         ],
         'merge': {
           'args': [