Reland "Enable field trial testing config for GuestViewCrossProcessFrames."

The original CL was reverted because it caused some bots to become flaky.
This was tracked on bug 747125 and should be fixed.

This is a reland of 8a8b48bb
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}

Bug: 655753
Change-Id: I59b128cf163be9cc565973f804811e5993ca01be

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

Change-Id: I59b128cf163be9cc565973f804811e5993ca01be
Reviewed-on: https://chromium-review.googlesource.com/591628Reviewed-by: Lucas Gadani <lfg@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491456}

testing/variations/fieldtrial_testing_config.json

@@ -1024,6 +1024,24 @@
             ]
         }
     ],
+    "GuestViewCrossProcessFrames": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "GuestViewCrossProcessFrames"
+                    ]
+                }
+            ]
+        }
+    ],
     "Html5ByDefault": [
         {
             "platforms": [