The initial support for low level exception handling in Wasm will not
support finally blocks. This decision is taken for both simplicity (
handling finallys is not straightforward if we want try blocks to yield
values), and lack of good use case (clang++ does not need them.) They
may be added in the future once we understand the implications of
having them.

BUG=

Review-Url: https://codereview.chromium.org/2336303002
Cr-Commit-Position: refs/heads/master@{#39393}