Add field trial for GWP-ASan PartitionAlloc

Land field trial for GWP-ASan PartitionAlloc support now that it's been
enabled on canary/dev.

Bug: 960532
Change-Id: Ide91a31547894edfbb5ba796f768deae88134ecc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614024
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Auto-Submit: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660597}

testing/variations/fieldtrial_testing_config.json

@@ -2321,6 +2321,22 @@
             ]
         }
     ],
+    "GwpAsanPartitionAlloc": [
+        {
+            "platforms": [
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "GwpAsanPartitionAlloc"
+                    ]
+                }
+            ]
+        }
+    ],
     "HTTPReallyBadFinal": [
         {
             "platforms": [