Convert the overlay scrollbar flag to use FeatureList

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 108 matching lines @@
     "args": ["--disable-slimming-paint-invalidation"]
   },
   {
     "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-threaded-compositing",
+    "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",
-             "--force-overlay-fullscreen-video", "--enable-overlay-scrollbars", "--enable-overscroll-notifications",
+             "--force-overlay-fullscreen-video", "--enable-overscroll-notifications",
              "--enable-fixed-layout", "--enable-viewport", "--disable-canvas-aa",
              "--disable-composited-antialiasing"]
   },
   {
     "prefix": "media-gpu-accelerated",
     "base": "http/tests/media/media-source",
     "args": ["--use-gpu-in-tests"]
   },
   {
     "prefix": "media-gpu-accelerated",
@@ skipping 343 matching lines @@
     "prefix": "print_browser",
     "base": "print_testharness",
     "args": ["--enable-print-browser"]
   },
   {
     "prefix": "experimental-canvas-features",
     "base": "fast/canvas-experimental",
     "args": ["--enable-experimental-canvas-features"]
   }
 ]