Clean up deprecated sources assignment filter in //chrome/installer

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

Bug: 1018739
Change-Id: I3b18cbad0acec25f6662e1e142225b46aac99614
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445871
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814316}

chrome/installer/util/BUILD.gn

@@ -7,13 +7,6 @@ import("//build/config/chrome_build.gni")
 
 import("//testing/test.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)
-
 # This file deliberately has no default "util" target so dependants have to
 # specify with the ":with_no_strings" or ":with_rc_strings" variants. Random
 # code that ends up getting linked into chrome proper should depend on the