This CL splits the definition of ValueType and its helper functions
into its own header file.

R=clemensh@chromium.org

Bug: v8:7570
Change-Id: I3aa776edb45839d7d38836e131df45732c685310
Reviewed-on: https://chromium-review.googlesource.com/1021810
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52758}