- Remove (one version of) InterpreterAssembler::LoadContextSlot in favor of the
  identical CodeStubAssembler::LoadContextElement.
- Use CodeStubAssembler::LoadNativeContext instead of doing the load manually.

R=rmcilroy@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2470253003
Cr-Commit-Position: refs/heads/master@{#40766}