Convert //ui to not depend on set_sources_assignment_filter()

set_sources_assignment_filter() is considered as a mis-feature
of gn (as it makes it difficult to reason about the BUILD.gn
files individually).

Convert BUILD.gn files below //ui to instead use conditional
to include source files only on the platformn where they are
used.

The conversion was done by using a modified version of gn that
display all files filtered by set_sources_assignment_filter()
and updating the BUILD.gn files manually to explicitly add the
source files only on the platform that used them.

This CL was uploaded by git cl split.

Bug: 1018739
Change-Id: I27b45e7f11292110fe046a7c6a20aac45fd6998c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930805Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718611}

ui/views/controls/webview/BUILD.gn

@@ -4,12 +4,16 @@
 
 import("//build/config/jumbo.gni")
 
+# Reset sources_assignment_filter for the BUILD.gn file to prevent
+# regression during the migration of Chromium away from the feature.
+# See build/no_sources_assignment_filter.md for more information.
+# TODO(crbug.com/1018739): remove this when migration is done.
+set_sources_assignment_filter([])
+
 jumbo_component("webview") {
   sources = [
     "unhandled_keyboard_event_handler.cc",
     "unhandled_keyboard_event_handler.h",
-    "unhandled_keyboard_event_handler_mac.mm",
-    "unhandled_keyboard_event_handler_win.cc",
     "web_contents_set_background_color.cc",
     "web_contents_set_background_color.h",
     "web_dialog_view.cc",
@@ -19,6 +23,14 @@ jumbo_component("webview") {
     "webview_export.h",
   ]
 
+  if (is_mac) {
+    sources += [ "unhandled_keyboard_event_handler_mac.mm" ]
+  }
+
+  if (is_win) {
+    sources += [ "unhandled_keyboard_event_handler_win.cc" ]
+  }
+
   defines = [ "WEBVIEW_IMPLEMENTATION" ]
 
   if (is_mac) {