cros: Delete legacy wallpaper directory

The only file remaining in the directory is
|wallpaper_manager_policy_browsertest.cc|, and these tests mainly test
the code path in |chrome_user_manager_impl|, so it makes sense to put
them together.

//ash/wallpaper does not rely on these tests. There's sufficient test
coverage in //ash for user policy and device policy wallpapers already.

In addition, remove //ash dependency to make it work under Mash.

Bug: NONE
Test: browser_tests
Change-Id: I187e9d34c69cfc63c8119c05886f96274a41dc8c
Reviewed-on: https://chromium-review.googlesource.com/978838Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545663}

chrome/browser/chromeos/login/users/wallpaper/OWNERS

-bshe@chromium.org
-wzang@chromium.org
-xdai@chromium.org
-
-# COMPONENT: UI>Shell>Wallpaper

chrome/test/BUILD.gn

@@ -1607,7 +1607,7 @@ test("browser_tests") {
         "../browser/chromeos/login/users/avatar/user_image_manager_browsertest.cc",
         "../browser/chromeos/login/users/avatar/user_image_manager_test_util.cc",
         "../browser/chromeos/login/users/avatar/user_image_manager_test_util.h",
-        "../browser/chromeos/login/users/wallpaper/wallpaper_manager_policy_browsertest.cc",
+        "../browser/chromeos/login/users/wallpaper_policy_browsertest.cc",
         "../browser/chromeos/login/webview_login_browsertest.cc",
         "../browser/chromeos/login/wizard_controller_browsertest.cc",
         "../browser/chromeos/net/network_portal_detector_impl_browsertest.cc",