Revert "Enable field trial testing config for GuestViewCrossProcessFrames."

This reverts commit 8a8b48bb.

Reason for revert: Speculative revert to fix broken WebUIWebViewBrowserTests, see http://crbug.com/747125

Original change's description:
> Enable field trial testing config for GuestViewCrossProcessFrames.
> 
> Bug: 655753
> Change-Id: I756aada2893846878657fe33b405bdb70b23df54
> Reviewed-on: https://chromium-review.googlesource.com/576207
> Commit-Queue: James MacLean <wjmaclean@chromium.org>
> Reviewed-by: Alexei Svitkine (slow) <asvitkine@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#487966}

TBR=asvitkine@chromium.org,lfg@chromium.org,wjmaclean@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 655753, 747125
Change-Id: Id7486ff811442a5fd5e639deaef152e4382a70bc
Reviewed-on: https://chromium-review.googlesource.com/580547Reviewed-by: Scott Little <sclittle@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488458}

testing/variations/fieldtrial_testing_config.json

@@ -1227,24 +1227,6 @@
             ]
         }
     ],
-    "GuestViewCrossProcessFrames": [
-        {
-            "platforms": [
-                "chromeos",
-                "linux",
-                "mac",
-                "win"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "GuestViewCrossProcessFrames"
-                    ]
-                }
-            ]
-        }
-    ],
     "Html5ByDefault": [
         {
             "platforms": [