Remove DiscardBeforeUnloadHelper class from the chrome binary.

This class has been created one year ago, but it has never been used so
far.

This CL removes it from the chrome binary. It is kept in the test
binary, so that it continue to be maintained

Whether to remove this class or not should be considered.

For the context, I was looking about WebContentsObserver and their
interactions with the BackForwardCache. Being unused/removed simplify
the question.

Bug: 877550, 511340
Change-Id: Iae4c523528a1ad307c91ae7cb53aa855094d3c0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768322
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690393}

chrome/browser/BUILD.gn

@@ -1499,8 +1499,6 @@ jumbo_split_static_library("browser") {
     "renderer_host/chrome_render_widget_host_view_mac_history_swiper.mm",
     "renderer_preferences_util.cc",
     "renderer_preferences_util.h",
-    "resource_coordinator/discard_before_unload_helper.cc",
-    "resource_coordinator/discard_before_unload_helper.h",
     "resource_coordinator/resource_coordinator_parts.cc",
     "resource_coordinator/resource_coordinator_parts.h",
     "resource_coordinator/session_restore_policy.cc",

chrome/test/BUILD.gn

@@ -1060,6 +1060,8 @@ if (!is_android) {
       "../browser/renderer_context_menu/spelling_menu_observer_browsertest.cc",
       "../browser/renderer_host/render_process_host_chrome_browsertest.cc",
       "../browser/repost_form_warning_browsertest.cc",
+      "../browser/resource_coordinator/discard_before_unload_helper.cc",
+      "../browser/resource_coordinator/discard_before_unload_helper.h",
       "../browser/resource_coordinator/discard_before_unload_helper_browsertest.cc",
       "../browser/resource_coordinator/local_site_characteristics_data_store_factory_browsertest.cc",
       "../browser/resource_coordinator/local_site_characteristics_data_unittest_utils.cc",