[Spellcheck] Fix field trial entry name for spell check experiment

The experiment name for the Windows native spell checker integration
does not match the name of the Chrome variation field trial
configuration entry. This changes the name in the field trial
configuration so it matches the Finch experiment name.

Bug: 978460
Change-Id: If10344fd9312535b5e8c1917cf6d3c7a6a80fd7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2356689
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798260}

testing/variations/fieldtrial_testing_config.json

@@ -8414,7 +8414,7 @@
             ]
         }
     ],
-    "WindowsHybridSpellCheck": [
+    "WindowsNativeSpellChecker": [
         {
             "platforms": [
                 "windows"