[ios] Rename FullscreenController testing config

Update it to match Google Finch config name

Bug: 790886
Change-Id: I1bf4e02619cdf334dcefe5ac620cbd321eb3f1a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398967
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806274}

testing/variations/fieldtrial_testing_config.json

@@ -2959,7 +2959,7 @@
             ]
         }
     ],
-    "FullscreenBrowserScope": [
+    "FullscreenController": [
         {
             "platforms": [
                 "ios"