[Context menu redesign] Follow-up to update the dialog's theme

Remove the scrim's background color because the theme already dims the
background.

Bug: 989330, 989336
Change-Id: I488e171a4d70317df4a5a0de59e281e3e350fb3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1744648
Commit-Queue: Sinan Sahin <sinansahin@google.com>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685354}

chrome/android/java/res/layout/revamped_context_menu.xml

@@ -6,7 +6,7 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@color/modal_dialog_scrim_color">
+    android:background="@android:color/transparent">
 
     <!-- We need 2 FrameLayouts here because the outer one is the scrim that covers the whole screen
          while the inner one is a frame for the ListView. We use this frame as a background for the