Partially implement WorkerFetchContext and WorkerThreadableLoadingContext and add virtual tests

third_party/WebKit/LayoutTests/VirtualTestSuites

 [
   {
     "prefix": "color_space",
     "base": "fast/canvas/color-space",
     "args": ["--enable-color-correct-rendering",
              "--enable-true-color-rendering"]
   },
   {
     "prefix": "gpu",
     "base": "fast/canvas",
@@ skipping 101 matching lines @@
     "prefix": "disable-spinvalidation",
     "base": "compositing",
     "args": ["--disable-slimming-paint-invalidation"]
   },
   {
     "prefix": "disable-spinvalidation",
     "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",
     "args": ["--disable-blink-features=ForeignFetch,InstalledApp,LinkServiceWorker,LongTaskObserver,SetRootScroller,WebUSB,WebVR,WebShare"]
   },
   {
     "prefix": "android",
     "base": "fullscreen",
     "args": ["--enable-features=OverlayScrollbar", "--enable-threaded-compositing",
              "--enable-fixed-position-compositing", "--enable-prefer-compositing-to-lcd-text",
              "--enable-composited-scrolling-for-frames", "--enable-gesture-tap-highlight", "--enable-pinch",
@@ skipping 293 matching lines @@
     "prefix": "new-remote-playback-pipeline",
     "base": "media/remoteplayback",
     "args": ["--enable-features=NewRemotePlaybackPipeline"]
   },
   {
     "prefix": "new-remote-playback-pipeline",
     "base": "media/controls",
     "args": ["--enable-features=NewRemotePlaybackPipeline"]
   }
 ]