Split the GN 'snapshot_toolchain' logic out into an include file.

Split the GN 'snapshot_toolchain' logic out into an include file.

We need to build parts of v8 with a toolchain that might be different
from both the default (target) toolchain and the regular host toolchain,
because we need the snapshot to have the same bit-width as the target.
V8's build defines a 'snapshot_toolchain' setting for this.

It turns out that we need the value of this toolchain to be exposed
to the Chromium build because some of the test targets (in browser_tests)
depend on d8 and need to be able to built using the same toolchain.

R=brett@chromium.org, jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1350223004

Cr-Commit-Position: refs/heads/master@{#30854}