[ThreadPool]: Update Add MayBlockWithoutDelay testing config to enable FixedMaxBestEffortTasks.

Bug: 1059767
Change-Id: I69880f5bb32cb6400138f1af0b2cf9da56a08c59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129013Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755467}

testing/variations/fieldtrial_testing_config.json

@@ -3243,13 +3243,17 @@
     "MayBlockWithoutDelay": [
         {
             "platforms": [
-                "windows",
-                "mac"
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
             ],
             "experiments": [
                 {
                     "name": "Enabled",
                     "enable_features": [
+                        "FixedMaxBestEffortTasks",
                         "MayBlockWithoutDelay"
                     ]
                 }