[Self Share] Fix missing include for SendTabToSelfSubMenuModelTest

M78 builders require include for base::ASCIIToUTF16.

Bug: 1007538
Change-Id: I2715b0a25d680e1ed17aa01c15dfa8228442a04d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824181
Auto-Submit: Kristi Park <kristipark@chromium.org>
Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699541}

chrome/browser/ui/send_tab_to_self/send_tab_to_self_sub_menu_model_unittest.cc

@@ -4,6 +4,7 @@
 
 #include "chrome/browser/ui/send_tab_to_self/send_tab_to_self_sub_menu_model.h"
 
+#include "base/strings/utf_string_conversions.h"
 #include "chrome/browser/sync/send_tab_to_self_sync_service_factory.h"
 #include "chrome/test/base/browser_with_test_window_test.h"
 #include "components/send_tab_to_self/send_tab_to_self_sync_service.h"