Add a note about compress="brotli" to apk_size_regressions.md

Change-Id: Ifa38ede1cba1a49dff0845d32a18f7de884b6b4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838929Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702996}

docs/speed/apk_size_regressions.md

@@ -113,7 +113,10 @@ for a description of binary size tools.**
 
 ### Growth is from Native Resources (pak files)
 
- * Ensure `compress="gzip"` is used for all `chrome:` pages.
+ * Ensure `compress="gzip"` or `compress="brotli"` is used for all
+   highly-compressible (e.g. text) resources.
+   * Brotli compresses more but is much slower to decompress. Use brotli only
+     when performance doesn't matter much (e.g. internals pages).
  * Look at the SuperSize reports from the trybot to look for unexpected
    resources, or unreasonably large symbols.