Fix name to be stable

Change-Id: I63536d743f3396f422fb9959eada4e0fec733a42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2528633
Auto-Submit: Ted Meyer <tmathmeyer@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#826383}

testing/variations/fieldtrial_testing_config.json

@@ -1938,7 +1938,7 @@
             ]
         }
     ],
-    "D3D11VideoDecoder": [
+    "D3D11VideoDecoderStable": [
         {
             "platforms": [
                 "windows"