Currently, looking up a lexical context variable requires looking up
the variable name and then checking its mode. This can be a bottleneck
in Runtime_DeclareGlobals, even when no lexical context variables are
declared.

R=rossberg@chromium.org
BUG=crbug:517778
LOG=N

Review URL: https://codereview.chromium.org/1281883002

Cr-Commit-Position: refs/heads/master@{#30075}