This CL creates global-context.cc and moves the GlobalContext
constructor impl to this new file. Preparatory refactoring for import
syntax.

This CL also removes one unused static method from GlobalContext
and changes two use-sites where Get() was used together with a static
accessor.

Drive-by: "Include what you use" for global-context.h

Bug: v8:9183
Change-Id: Iafd877d03af9ad65b1c99ebd9743be64192f45c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1649790
Commit-Queue: Simon Zünd <szuend@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62081}