Requesting reservation of a wasm section for experimentation with
storing source code meta information, such as source code comments,
and also extra inform on presentation of the AST such an `if-block`
pattern being presented as a `when` operation.

The wasm design already defines unrecognized sections to be ignored,
and this reserved section is ignored. This section is only intended to
hold source code meta information and to have no effect on code
execution.

With wasm going live (behind a flag) on v8, I would also like to be
able to give people something to play with in terms of the deployed
binary code being a useful source code. It's all experimental, but I
understand the entire binary format that V8 is currently using is
basically a throwaway, and that the working strategy is to get
something running and then revisit format decisions.

I would like a fixed reserved section number to avoid potential
clashes with other projects - although I am not aware of any other
calls for addition sections beyond the need for debug info. If a fixed
number is not acceptable, then could this patch alternatively ignore
all unrecognized sections and perhaps add the section size to them
all - something which is already noted todo in the design document?

BUG=

Review URL: https://codereview.chromium.org/1565693002

Cr-Commit-Position: refs/heads/master@{#33165}