A stack overflow can be thrown by JSEntryStub, which means the
thread-in-wasm flag will not have the expected value. To accommodate
this, we now clear the flag during exceptional returns if it is set.

Bug: chromium:834624
Change-Id: I8359af79886ab98dfecc2fb39ca19118b7fa38eb
Reviewed-on: https://chromium-review.googlesource.com/1019570Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52891}