Clean up VizForWebView in field trial testing config

This CL actualized the feature list to match with server config. This
doesn't change any actual state of the code, UseSkiaRenderer is forced
in code for VizForWebView and UseSkiaForGLReadback doesn't exist
anymore.

Bug: 805739
Change-Id: Id36982afdc9547a569a23249b4c7b97610b5d197
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2576920Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834303}

testing/variations/fieldtrial_testing_config.json

@@ -7685,11 +7685,9 @@
             ],
             "experiments": [
                 {
-                    "name": "VizForWebView",
+                    "name": "Enabled",
                     "enable_features": [
                         "EnableSharedImageForWebview",
-                        "UseSkiaForGLReadback",
-                        "UseSkiaRenderer",
                         "VizForWebView"
                     ]
                 }