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Document and allow the [Serializable] and [Transferable] IDL extended
attributes. They don't influence code generation (custom code is
required), but should be allowed if the spec has them so that
spec/tests/code can match.

Add those attributes to .idl files that need them, and also clean up a
few other IDL extended attribute documentation nits.

Change-Id: I241d257c2d8cd4eacfb32354772daa54002cf878
Reviewed-on: https://chromium-review.googlesource.com/c/1255595
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595985}