[NativeFS] Changed Drag & Drop from test feature to experimental

Change NativeFS Drag & Drop from a 'test' runtime enabled feature to an
'experimental' runtime enabled feature. This will enable NativeFS drag
and drop by default if enable-experimental-web-platform-features flag
is set.

Bug: 1080811
Change-Id: Ie3fcc4a28f22a3287cf64607b90e44a0f7767607
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2356188Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Hazim Mohamed <hazimmohamed@google.com>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Hazim Mohamed <hazimmohamed@google.com>
Cr-Commit-Position: refs/heads/master@{#798186}

content/browser/native_file_system/native_file_system_drag_drop_browsertest.cc

@@ -38,10 +38,6 @@ class NativeFileSystemDragDropBrowserTest : public ContentBrowserTest {
     // Enable experimental web platform features to enable write access.
     command_line->AppendSwitch(
         switches::kEnableExperimentalWebPlatformFeatures);
-    // TODO(hazimmohamed): Remove this switch when NativeFileSystemDragAndDrop
-    // becomes part of kEnableExperimentalWebPlatformFeatures.
-    command_line->AppendSwitchASCII(switches::kEnableBlinkFeatures,
-                                    "NativeFileSystemDragAndDrop");
   }
 
   void TearDown() override {

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1211,7 +1211,7 @@
     },
     {
       name: "NativeFileSystemDragAndDrop",
-      status: "test"
+      status: "experimental"
     },
     {
       name: "NativeIO",