This CL
- removes InternalizeStringIfExists/LookupStringIfExists
- makes the distinction between hash_field and hash clear to AstRawString

Bug: 
Change-Id: Ia98c2236be4154a7db2741f2cf73681cfdcf03c7
Reviewed-on: https://chromium-review.googlesource.com/532954
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45902}