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Don't use sources_assignment_filters in //c/browser/extensions/BUILD.gn

set_sources_assignment_filter() causes files to be magically excluded
from sources lists based on path (e.g. *_win*). It is considered a
misfeature of gn, see bug or docs/no_sources_assignment_filter.md

Convert BUILD.gn to use conditionals to include source files only on
the platform where they are used.

Bug: 1018739, 1096762
Test: compiles

Change-Id: Ic5563a3558d9bdc8b9bc22bb6275d869a0985c5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303463
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789668}