Tweak platform conditional in patch to support cross builds

[iskunk]

In disable-domain-reliability.patch, a conditional block is added to allow a script in the domain_reliability component to accept a null device as input (NUL on Windows, /dev/null on Linux).

However, the condition is predicated on is_win, which translates to "is this build for Windows?" When I cross-compile ungoogled-chromium on Linux for Windows, I get this:

[1474/66598] ACTION //components/domain_reliability:bake_in_configs(//build/toolchain/win:win_clang_x64)
1757 FAILED: gen/components/domain_reliability/baked_in_configs.cc 
1758 python3 ../../components/domain_reliability/bake_in_configs.py --file-list nul --output gen/components/domain_reliability/baked_in_configs.cc
1759 Traceback (most recent call last):
1760   File "/home/build/u-c-w/cross-build/build/src/out/Default/../../components/domain_reliability/bake_in_configs.py", line 597, in <module>
1761     sys.exit(main())
1762              ^^^^^^
1763   File "/home/build/u-c-w/cross-build/build/src/out/Default/../../components/domain_reliability/bake_in_configs.py", line 549, in main
1764     json_files = read_json_files_from_file(opts.file_list)
1765                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1766   File "/home/build/u-c-w/cross-build/build/src/out/Default/../../components/domain_reliability/bake_in_configs.py", line 483, in read_json_files_from_file
1767     with open(list_file, 'r') as f:
1768          ^^^^^^^^^^^^^^^^^^^^
1769 FileNotFoundError: [Errno 2] No such file or directory: 'nul'

This PR changes the condition to host_os == "win" (i.e. "are we building on Windows?"), which I believe is what was intended. This allows the cross build to succeed.