This is a follow-up to 2d281e71 and prevents bailouts on empty
script contexts in Crankshaft, which don't need allocation. Only
non-empty script contexts should cause a bailout.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1063373003

Cr-Commit-Position: refs/heads/master@{#27649}