Block scopes don't need any special treatment here (it's unclear
to me why they ever did). And the has_forced_context_allocation() check
seems, according to our tests, to only have been necessary for proper
handling of 'with' scopes. This patch instead uses the "is_used" bit
to keep track of variables that are accessed from within a with.

R=neis@chromium.org

Review-Url: https://codereview.chromium.org/2220293003
Cr-Commit-Position: refs/heads/master@{#38505}