Click to Open PDF: Add to fieldtrial_testing_config.json

Bug: 737787
Change-Id: Ib967821fd071f718d72e9a300809d33070e9729c
Reviewed-on: https://chromium-review.googlesource.com/938957Reviewed-by: Anthony LaForge <laforge@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539947}

testing/variations/fieldtrial_testing_config.json

@@ -2397,6 +2397,25 @@
             ]
         }
     ],
+    "PDFClickToOpen": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "ClickToOpenPDFPlaceholder"
+                    ]
+                }
+            ]
+        }
+    ],
     "PWAFullCodeCache": [
         {
             "platforms": [