Only compile notification test code into tests

The file mock_itoastnotification.cc was compiled for the
chrome.dll binary instead of being in the test_support section
with the test-only code.

Noticed in jumbo build experiments since it ended up in a larger
obj file where the code was no longer ignored as before, ending
with a linker error because of inconsistent settings.

Bug: 746957
Change-Id: Ibf259cb729889f26226ff87b068fa1051898b11d
Reviewed-on: https://chromium-review.googlesource.com/901627
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534425}

chrome/browser/BUILD.gn

@@ -2899,8 +2899,6 @@ split_static_library("browser") {
       "downgrade/user_data_downgrade.cc",
       "downgrade/user_data_downgrade.h",
       "first_run/upgrade_util.cc",
-      "notifications/mock_itoastnotification.cc",
-      "notifications/mock_itoastnotification.h",
       "notifications/notification_image_retainer.cc",
       "notifications/notification_image_retainer.h",
       "notifications/notification_template_builder.cc",
@@ -4758,6 +4756,13 @@ static_library("test_support") {
     ]
   }
 
+  if (is_win) {
+    sources += [
+      "notifications/mock_itoastnotification.cc",
+      "notifications/mock_itoastnotification.h",
+    ]
+  }
+
   if (enable_app_list) {
     sources += [
       "ui/app_list/test/chrome_app_list_test_support.cc",