Hide RequestInit.keepalive in the experimental flag

The logic is unintentionally exposed and this CL fixes that.

Bug: 810466
Change-Id: I46091b1ee1fb787195ca9feb7be939df1c5bc258
Reviewed-on: https://chromium-review.googlesource.com/917361Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536649}

content/browser/renderer_host/render_process_host_browsertest.cc

@@ -148,6 +148,11 @@ class RenderProcessHostWithKeepAliveOptionEnabledTest
     RenderProcessHostTest::SetUp();
   }
 
+  void SetUpCommandLine(base::CommandLine* command_line) override {
+    command_line->AppendSwitch(
+        switches::kEnableExperimentalWebPlatformFeatures);
+  }
+
  private:
   base::test::ScopedFeatureList feature_list_;
 };

third_party/WebKit/Source/core/fetch/RequestInit.cpp

@@ -105,9 +105,11 @@ RequestInit::RequestInit(ExecutionContext* context,
   if (exception_state.HadException())
     return;
 
-  keepalive_ = h.Get<IDLBoolean>("keepalive");
-  if (exception_state.HadException())
-    return;
+  if (RuntimeEnabledFeatures::FetchRequestKeepaliveEnabled()) {
+    keepalive_ = h.Get<IDLBoolean>("keepalive");
+    if (exception_state.HadException())
+      return;
+  }
 
   auto v8_body = h.Get<IDLPassThrough>("body");
   if (exception_state.HadException())