Add BUILD.gn to build brotli

[harryterkelsen]

Brotli is needed to support .woff2 and .woff fonts on the web. woff2 is the default format served by Google Fonts.

See also flutter/engine#41282

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• 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.
• All existing and new tests are passing.

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[chinmaygarde]

Can we land this?

[mdebbar]

Is there a reason this has to be added in the buildroot repo? Can we put it in the engine repo like this?

[chinmaygarde]

No. That seems like a better place for this in fact. It is being DEPSed to the flutter/third_party though right?

[chinmaygarde]

@hterkelsen: Are you planning on taking this to flutter/third_party or leaving it here?

[chinmaygarde]

Closing this as stale.