harmony-scoping: Implement debugger support for script scope.

We add a new ScopeType, ScopeType.Script. The scope with
ScopeType.Script is always present in the scope chain (ScopeIterator
fakes it if neededi - i.e. if ScriptContext for a script has not been
allocated since that script has no lexical declarations).
ScriptScope reflects ScriptContextTable.

R=yurys@chromium.org,yangguo@chromium.org
BUG=v8:3690
LOG=N

Review URL: https://codereview.chromium.org/726643002

Cr-Commit-Position: refs/heads/master@{#25383}