Add download button to MediaDocument on Android.

For a MediaDocument on android, it is not easy
to long press the element and download audio.
This change adds a download button to the MediaDocument on android.
This facilitates user to save the media file locally.

UI
Review: https://groups.google.com/a/google.com/forum/#!msg/chrome-ui-review/XzyQ2QsVyRE/0x5jU-KECwAJ
Mocks: https://folio.googleplex.com/chrome-ux/mocks/065-downloads/06-media/022216_Buttons

BUG=590043

Review URL: https://codereview.chromium.org/1780043002

Cr-Commit-Position: refs/heads/master@{#385607}
diff --git a/third_party/WebKit/LayoutTests/VirtualTestSuites b/third_party/WebKit/LayoutTests/VirtualTestSuites
index e243fe1..f9f5f74 100644
--- a/third_party/WebKit/LayoutTests/VirtualTestSuites
+++ b/third_party/WebKit/LayoutTests/VirtualTestSuites
@@ -98,6 +98,11 @@
              "--disable-composited-antialiasing"]
   },
   {
+    "prefix": "android",
+    "base": "media/mediadocument",
+    "args": ["--enable-features=MediaDocumentDownloadButton"]
+  },
+  {
     "prefix": "stable",
     "base": "fast/css3-text/css3-text-decoration/stable",
     "args": ["--stable-release-mode"]