Add deep scanning for Advanced Protection users to fieldtrial_testing_config

This launch has been rolled out to all Canary/Dev users for a few days,
and we don't see any stability impact. Since this code only triggers on
a small proportion of downloads from Advanced Protection users (~50,000
users), we don't expect our code paths have actually been executed much
in the wild. We can be reasonably confident we aren't causing problems
for non-APP users, however.

Change-Id: I9a3cf16459d839623ac8417a032dce031cbed798
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402134
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808445}

testing/variations/fieldtrial_testing_config.json

@@ -2004,6 +2004,23 @@
             ]
         }
     ],
+    "DeepScanningForAppUsers": [
+        {
+            "platforms": [
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Prompt for scanning",
+                    "enable_features": [
+                        "SafeBrowsingPromptAppForDeepScanning"
+                    ]
+                }
+            ]
+        }
+    ],
     "DefaultEnableOopRasterization": [
         {
             "platforms": [