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 429b4f5037eabce130401c5dba909b01b18b2bb5..4c4d34220a8715555881194424d561218f306f7e 100644
--- a/third_party/WebKit/LayoutTests/VirtualTestSuites
+++ b/third_party/WebKit/LayoutTests/VirtualTestSuites
@@ -119,6 +119,16 @@
     "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",
     "args": ["--disable-blink-features=ForeignFetch,InstalledApp,LinkServiceWorker,LongTaskObserver,SetRootScroller,WebUSB,WebVR,WebShare"]