Default parameters have additional declaration block scopes inserted
around them when something in the function scope calls eval. This
patch sets the parent scope of the expressions introduced due to
those defaults to the new block scope.

R=adamk
BUG=chromium:616386

Review-Url: https://codereview.chromium.org/2077283004
Cr-Commit-Position: refs/heads/master@{#37198}