While we properly handled scopes of initializers in destructured parameters,
we never did the right thing for computed properties. This patch fixes that
by factoring out PatternRewriter's scope rewriting logic and calls it
for the computed property case.

BUG=chromium:620119

Review-Url: https://codereview.chromium.org/2084103002
Cr-Commit-Position: refs/heads/master@{#37228}