Add PartitionAllocGigaCageMobie to fieldtrial_testing_config.json

Bug: 1086388
Change-Id: Iadcb930d49e905dcf94511f3fb213e06f0cce547
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505704Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#826715}

testing/variations/fieldtrial_testing_config.json

@@ -5109,7 +5109,7 @@
             ],
             "experiments": [
                 {
-                    "name": "Enabled",
+                    "name": "Enabled_Mobile",
                     "enable_features": [
                         "PartitionAllocGigaCage"
                     ]