Started generating the windows platform for macrobenchmarks

[gaaclarke]

Fixes execution of the windows shadowed text benchmark.

Example failure: https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8673758721855002449/+/u/run_windows_text_shadow_perf__timeline_summary/stdout

successful run: https://ci.chromium.org/ui/p/flutter/builders/staging.shadow/Windows%20windows_text_shadow_perf__timeline_summary/1/overview

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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
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Code Review

This pull request updates the createTextShadowPerfTest function in perf_tests.dart to accept an optional createPlatforms parameter, which is passed to the PerfTest constructor. Additionally, the Windows text shadow performance task is updated to pass ['windows'] as the createPlatforms argument. I have no feedback to provide.