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This adds a section to explain how to out-of-line methods which return
or accept new types. This isn't generally necessary for implementation
code, but this is important for external-facing classes (such as
WebView's glue layer).

Fixed: 1112420
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Ia5f835663780c50e1d1601e969a5a6d550e9fbf5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493430
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820381}