Ship keepalive on Fetch API

Intent to ship thread: https://groups.google.com/a/chromium.org/forum/#!topic/Blink-dev/8vqcABTnDF4

Bug: 695939
Change-Id: I917909f4ee0e5cf2967ac4ed44b137e9a9b6e2a1
Reviewed-on: https://chromium-review.googlesource.com/923442Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537564}

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

@@ -779,6 +779,7 @@ interface Request
     getter destination
     getter headers
     getter integrity
+    getter keepalive
     getter method
     getter mode
     getter redirect

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

@@ -749,6 +749,7 @@ Starting worker: resources/global-interface-listing-worker.js
 [Worker]     getter destination
 [Worker]     getter headers
 [Worker]     getter integrity
+[Worker]     getter keepalive
 [Worker]     getter method
 [Worker]     getter mode
 [Worker]     getter redirect

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

@@ -4532,6 +4532,7 @@ interface Request
     getter destination
     getter headers
     getter integrity
+    getter keepalive
     getter method
     getter mode
     getter redirect

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

@@ -744,6 +744,7 @@ Starting worker: resources/global-interface-listing-worker.js
 [Worker]     getter destination
 [Worker]     getter headers
 [Worker]     getter integrity
+[Worker]     getter keepalive
 [Worker]     getter method
 [Worker]     getter mode
 [Worker]     getter redirect

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -415,7 +415,7 @@
     },
     {
       name: "FetchRequestKeepalive",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "FileSystem",