This updates the type of contexts to NativeContext instead of Context,
namely on GetFunctionRealm(), GetCreationContext(), and
JSGlobalObject::native_context.

They should be semantically NativeContexts, but the return type hides
the underlying NativeContext, and causes its user to cast the context
to native.

Change-Id: I2f234b0df8c2dcaeab25cb543e09d80d12ca7369
Reviewed-on: https://chromium-review.googlesource.com/c/1469541Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59543}