Add PDFViewerUpdate field trial config.

Bug: 1048812
Change-Id: I7ddf909e0c5179f758230b9bd93734e015a6c45f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436273Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812214}

testing/variations/fieldtrial_testing_config.json

@@ -5256,6 +5256,24 @@
             ]
         }
     ],
+    "PdfViewerUpdate": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "PdfViewerUpdate"
+                    ]
+                }
+            ]
+        }
+    ],
     "PerProcessReclaim": [
         {
             "platforms": [