Generate param traits size methods for IPC files in content/ (and traits it depends on).

This allows us to reuse these traits in Mojo if needed.

I didn't add GetSize to every single trait in content/ and below; just enough to get the content message generator to compile with the size traits.

NOTRY=true

Review-Url: https://codereview.chromium.org/1966983003
Cr-Commit-Position: refs/heads/master@{#393058}
39 files changed