Reland "Add a virtual-content element that uses display locking to make content low cost when off-screen."

This is a reland of f1ddcc975080503c8ff089ed53d2d4758a9c3ff4

This marks some tests as slow on Mac 10.13 dbg. Once I know how slow,
I can try fix this.

Original change's description:
> Add a virtual-content element that uses display locking to make content
> low cost when off-screen.
>
> Exposes .mjs files as http-available files for web_tests. This required
> adding .mjs to the mime config and various things in BUILD files.
>
> Add .mjs as javascript to the mime configs for webtest server.
>
> Delete old virtual-scroller code.
>
> Bug: 979108
> Change-Id: Idfde0bb23263b571fc3abf98845bb7d0327baf07
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1680170
> Commit-Queue: Fergal Daly <fergal@chromium.org>
> Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
> Reviewed-by: Kent Tamura <tkent@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#676308}

BUG=984003,979108
TBR=domenic@chromium.org

Change-Id: Ieec1c13c3ceaba28ec7bfb32fa948354bffd28a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1702003
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677633}

diff --git a/BUILD.gn b/BUILD.gn
index 60a5332..d15e139 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -1039,6 +1039,7 @@
       "//testing/buildbot/filters:blink_web_tests_filter",
       "//third_party/blink/public:blink_devtools_frontend_resources_files",
       "//third_party/blink/public/mojom:mojom_platform_js_data_deps",
+      "//third_party/blink/renderer/core/script:js_files_for_web_tests",
       "//third_party/mesa_headers",
       "//tools/imagediff",
     ]