This involved adding a count_ member to SloppyBlockFunctionMap, so
to avoid making DeclarationScope larger, this patch makes the
creation of the map lazy, thus reducing the size of DeclarationScope
by several words in the process.

BUG=chromium:688567

Change-Id: If9a9eb2ccc01690fe10edadb3aa9625454ff4a19
Reviewed-on: https://chromium-review.googlesource.com/448701
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43558}