Because the Scope will be optimized away by the call to
FinalizeBlockScope in the case where there are no lexical
declarations in the block, this should have no effect on
anything downstream from the Parser, and simply removes
duplicate parsing code.

Due to the change from ParseStatement to ParseStatementListItem,
this will result in slightly different error messages for
lexical declarations in sloppy mode (until those are shipped).

R=littledan@chromium.org, rossberg@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1433743005

Cr-Commit-Position: refs/heads/master@{#31966}