ios: Add FastApplicationWillTerminate to skip thread shutdown.

_chromeMain.reset() is a blocking call that regularly causes
 applicationWillTerminate to fail after a 5s delay. Experiment with skipping this shutdown call.

Bug: 1328891
Change-Id: Ia2bbdc4b07f60cca577e934cdf101c107efb705b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3664078
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1007572}
NOKEYCHECK=True
GitOrigin-RevId: 85b498b0c28c769467da78a759a4d62eb3d3da3a
diff --git a/variations/fieldtrial_testing_config.json b/variations/fieldtrial_testing_config.json
index cd1b46d..e8897a9 100644
--- a/variations/fieldtrial_testing_config.json
+++ b/variations/fieldtrial_testing_config.json
@@ -4389,6 +4389,21 @@
             ]
         }
     ],
+    "IOSFastApplicationWillTerminate": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "FastApplicationWillTerminate"
+                    ]
+                }
+            ]
+        }
+    ],
     "IOSKeepsRenderProcessAlive": [
         {
             "platforms": [