Modifies V8 to be able to parse the exception section (defining
exception types), when the experimental_wasm_eh flag is true.

Bug: v8:6577
Change-Id: I5d8b3fddaf5b0dec6b14ddd0992f9fb883e8dc90
Reviewed-on: https://chromium-review.googlesource.com/561757
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46539}