Move MockDownloadController to //chrome/browser:test_support

The Android MockDownloadController was added in:
https://codereview.chromium.org/1217223006

However, it seems to have been incorrectly added to our prod target
instead of something only for tests. This is unnecessary since
it is never used in prod.

This CL moves it to //chrome/browser:test_support, which is already
depsed in for the target that the single unit test that uses it
(DownloadResourceThrottleTest) is part of.

BUG=None

Change-Id: I70e7c06e603d38f858aadcffb47ac9bbd256e656
Reviewed-on: https://chromium-review.googlesource.com/471927Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#462996}

chrome/browser/BUILD.gn

@@ -2755,8 +2755,6 @@ split_static_library("browser") {
       "android/download/intercept_download_resource_throttle.h",
       "android/download/items/offline_content_aggregator_factory_android.cc",
       "android/download/items/offline_content_aggregator_factory_android.h",
-      "android/download/mock_download_controller.cc",
-      "android/download/mock_download_controller.h",
       "android/download/ui/thumbnail_provider.cc",
       "android/download/ui/thumbnail_provider.h",
       "android/favicon_helper.cc",
@@ -4450,7 +4448,12 @@ static_library("test_support") {
     "//ui/gfx",
   ]
 
-  if (!is_android) {
+  if (is_android) {
+    sources += [
+      "android/download/mock_download_controller.cc",
+      "android/download/mock_download_controller.h",
+    ]
+  } else {
     sources += [
       "chooser_controller/mock_chooser_controller.cc",
       "chooser_controller/mock_chooser_controller.h",