Move tests for sendBeacon to a separate dedicated directory for clearer ownership

Not only because of the ownership clarification, but it's just good to
have a better home for them since they're closer to XHR and Fetch API.
It's not so reasonable to place them in navigation/ just because they
belongs to window.navigator.

The virtual test virtual/stable/http/tests/navigation/ is kept for a
different reason.

Moved redirection-respond.php to http/tests/resources as it's not
specific to the tests under http/tests/navigation/

BUG=682571,619662,490015,694958

Review-Url: https://codereview.chromium.org/2702113003
Cr-Commit-Position: refs/heads/master@{#452408}
diff --git a/third_party/WebKit/LayoutTests/VirtualTestSuites b/third_party/WebKit/LayoutTests/VirtualTestSuites
index 7d3b9e801..6a8828f 100644
--- a/third_party/WebKit/LayoutTests/VirtualTestSuites
+++ b/third_party/WebKit/LayoutTests/VirtualTestSuites
@@ -90,6 +90,11 @@
   },
   {
     "prefix": "stable",
+    "base": "http/tests/sendbeacon",
+    "args": ["--stable-release-mode"]
+  },
+  {
+    "prefix": "stable",
     "base": "animations-unprefixed",
     "args": ["--stable-release-mode"]
   },