Enable SpdyProxiesRequestsDelayable by default in developer builds

Change-Id: Ia4ad8b4647432352928e23e5cbc6e4c29cfa0443
Bug: 826558
Reviewed-on: https://chromium-review.googlesource.com/1035444Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555272}

testing/variations/fieldtrial_testing_config.json

@@ -3649,6 +3649,25 @@
             ]
         }
     ],
+    "SpdyProxiesRequestsDelayable": [
+        {
+            "platforms": [
+                "android",
+                "win",
+                "mac",
+                "chromeos",
+                "linux"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SpdyProxiesRequestsDelayable"
+                    ]
+                }
+            ]
+        }
+    ],
     "SpeculativeLaunchServiceWorker": [
         {
             "platforms": [