Making new nodes inside of exception-handled blocks fiddles around with the
current environment to merge the exception paths. In particular, the current
environment pointer is mutated. This patch ensures that when we merge the fast
and slow paths of the LdaContextLookup, we actually merge the correct
environment and do not accidentally merge the exceptional environment.

BUG=chromium:651394

Review-Url: https://codereview.chromium.org/2379043002
Cr-Commit-Position: refs/heads/master@{#39878}