Clean up deprecated sources assignment filter in //chrome

Remove deprecated_default_sources_assignment_filter and replace
it with explicit is_win, is_mac, etc. sections.

Bug: 1018739
Change-Id: If292e7c9a584afabfa11ca4484f330ae767eaa72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443289Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814315}

chrome/BUILD.gn

@@ -45,13 +45,6 @@ if (is_android) {
   import("//third_party/icu/config.gni")
 }
 
-# This file depends on the legacy global sources assignment filter. It should
-# be converted to check target platform before assigning source files to the
-# sources variable. Remove this import and set_sources_assignment_filter call
-# when the file has been converted. See https://crbug.com/1018739 for details.
-import("//build/config/deprecated_default_sources_assignment_filter.gni")
-set_sources_assignment_filter(deprecated_default_sources_assignment_filter)
-
 declare_args() {
   # Indicates whether keystone registration framework should be enabled (see
   # action("keystone_registration_framework") below).  There are some tests