The most common use of {SaveContext} is to allocate this object, then
immediately set the context of the isolate to another Context. Thus
introduce a second class called "SaveAndSwitchContext" which implements
exactly that.

R=mstarzinger@chromium.org

Bug: v8:8562
Change-Id: I2fca1eadd909a7afe035316ded934624273f2e21
Reviewed-on: https://chromium-review.googlesource.com/c/1448319Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59323}