Partially implement WorkerFetchContext and WorkerThreadableLoadingContext and add virtual tests

third_party/WebKit/LayoutTests/VirtualTestSuites

Index: third_party/WebKit/LayoutTests/VirtualTestSuites
diff --git a/third_party/WebKit/LayoutTests/VirtualTestSuites b/third_party/WebKit/LayoutTests/VirtualTestSuites
index 8f5707b4802af24e0edd48be480c0a4e09f214b4..bcc1a8107c73d20472320cf0f80f93268381f9af 100644
--- a/third_party/WebKit/LayoutTests/VirtualTestSuites
+++ b/third_party/WebKit/LayoutTests/VirtualTestSuites
@@ -118,6 +118,16 @@
     "base": "paint",
     "args": ["--disable-slimming-paint-invalidation"]
   },
+  {
+    "prefix": "off-main-thread-fetch",
+    "base": "http/tests/fetch/workers",
+    "args": ["--enable-features=OffMainThreadFetch"]
+  },
+  {
+    "prefix": "off-main-thread-fetch",
+    "base": "http/tests/xmlhttprequest/workers",
+    "args": ["--enable-features=OffMainThreadFetch"]
+  },
   {
     "prefix": "origin-trials-runtimeflags-disabled",
     "base": "http/tests/origin_trials/webexposed",