[NativeFS] Introduce a browser test for native file system drag and drop

Introduces a browser test that implements end-to-end testing of
the DataTransferItem.getAsFileSystemHandle() method. It contains two
tests: one for dropped files and one for dropped directories.

Bug: 1080811
Change-Id: Ie64096539a732c0a2d101ca98bf9e86fcc9190ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339945
Commit-Queue: Hazim Mohamed <hazimmohamed@google.com>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797986}

content/test/BUILD.gn

@@ -1010,6 +1010,7 @@ test("content_browsertests") {
     "../browser/message_port_provider_browsertest.cc",
     "../browser/mojo_sandbox_browsertest.cc",
     "../browser/native_file_system/file_system_chooser_browsertest.cc",
+    "../browser/native_file_system/native_file_system_drag_drop_browsertest.cc",
     "../browser/native_file_system/native_file_system_file_writer_impl_browsertest.cc",
     "../browser/navigation_browsertest.cc",
     "../browser/navigation_mhtml_browsertest.cc",