When looking up variables in the ScopeInfo, we did a linear scan of the
ScopeInfo. Since that's unacceptably slow, a context slot cache was added
that would speed up repeated lookups of the same variable.

Instead, just always fully convert the ScopeInfo into scopes, so they can
lookup variables without scanning the ScopeInfo.

This also allows for removing the now unused ContextSlotCache.

R=adamk@chromium.org,verwaest@chromium.org,marja@chromium.org
BUG=v8:5315

Review-Url: https://codereview.chromium.org/2280933002
Cr-Commit-Position: refs/heads/master@{#38953}