bind-gen: Introduce a new SequenceNode

Introduces a new SequenceNode and replaces existing use cases of
SymbolScopeNode with the SequenceNode if appropriate.

The implementation of SequenceNode and SymbolScopeNode is
incomplete, and the follow-up patches will complete the two
classes.

Bug: 839389
Change-Id: I6d578ffe94c386757e76764a4d02221ef0847f51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1946176
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720790}
4 files changed