[Spellcheck] Add field trial entry for hybrid spell checking experiment

The Windows native spell checker integration is almost ready to launch.

This CL adds an entry to field trial testing data in preparation for
the dev experiment.

Bug: 978460, 463364,
Change-Id: I3e5fd4af68d94445afa1cd1208ee65d2b5d742ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001243
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738340}

testing/variations/fieldtrial_testing_config.json

@@ -7772,6 +7772,22 @@
             ]
         }
     ],
+    "WindowsHybridSpellCheck": [
+        {
+            "platforms": [
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "WinUseBrowserSpellChecker",
+                        "WinUseHybridSpellChecker"
+                    ]
+                }
+            ]
+        }
+    ],
     "WindowsSensorAPIs": [
         {
             "platforms": [