In Torque, we have several global singleton classes. Using the contextual
variable pattern instead of passing around pointers in random places
makes the code more readable.
This CL does this for TypeOracle, we plan to do it for more classes in
the future.

Bug: v8:7754
Change-Id: Ib744b476ce51a4932c52274b2210149515f1663d
Reviewed-on: https://chromium-review.googlesource.com/1078729
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53444}