PreParser is already capable of handling natives, because
ParseV8Intrinsic was moved to ParserBase. There's no reason to force
eager parsing when natives are allowed.

R=nikolaos@chromium.org, mstarzinger@chromium.org
BUG=v8:5398

Review-Url: https://codereview.chromium.org/2342133003
Cr-Commit-Position: refs/heads/master@{#39501}