Checking for native context is faster than checking for global object. Additionally it speeds up the case were it actually is the native context, while not slowing down the alternative case. The bootstrapper only needs to access the native context from the native context, so this avoids the expensive fallback.

BUG=

Review URL: https://codereview.chromium.org/1214903017

Cr-Commit-Position: refs/heads/master@{#29423}