Enable NetInfo saveData attribute by default

blink-dev i2s thread with 3 LGTMs:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/AgonJunvfIE

Bug: 772566
Change-Id: I09178ae913a513de7dc646b1dd43cdffafe1b3c5
Reviewed-on: https://chromium-review.googlesource.com/823252Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523798}

third_party/WebKit/LayoutTests/platform/mac/virtual/stable/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt

@@ -620,6 +620,7 @@ interface NetworkInformation : EventTarget
     getter effectiveType
     getter onchange
     getter rtt
+    getter saveData
     method constructor
     setter onchange
 interface Notification : EventTarget

third_party/WebKit/LayoutTests/virtual/stable/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt

@@ -620,6 +620,7 @@ interface NetworkInformation : EventTarget
     getter effectiveType
     getter onchange
     getter rtt
+    getter saveData
     method constructor
     setter onchange
 interface Notification : EventTarget

third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-dedicated-worker-expected.txt

@@ -600,6 +600,7 @@ Starting worker: resources/global-interface-listing-worker.js
 [Worker]     getter effectiveType
 [Worker]     getter onchange
 [Worker]     getter rtt
+[Worker]     getter saveData
 [Worker]     method constructor
 [Worker]     setter onchange
 [Worker] interface Notification : EventTarget

third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -3760,6 +3760,7 @@ interface NetworkInformation : EventTarget
     getter effectiveType
     getter onchange
     getter rtt
+    getter saveData
     method constructor
     setter onchange
 interface Node : EventTarget

third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-shared-worker-expected.txt

@@ -595,6 +595,7 @@ Starting worker: resources/global-interface-listing-worker.js
 [Worker]     getter effectiveType
 [Worker]     getter onchange
 [Worker]     getter rtt
+[Worker]     getter saveData
 [Worker]     method constructor
 [Worker]     setter onchange
 [Worker] interface Notification : EventTarget

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -683,7 +683,7 @@
     },
     {
       name: "NetInfoSaveData",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "NetworkService",