Changes include:
 - Adding V8_EXPORT macro for SnapshotCreator
 - Removing outdated DCHECKs.
 - Allow nullptr as external reference. This required a...
 - Refactoring of hashmaps used by the serializer.
 - Remove external references for counters. These are not used
   anywhere for isolates that are being serialized.
 - Put template infos into the partial snapshot cache.
 - Remove unnecessary presubmit check for external references.
   mksnapshot crashes if external references are missing.

R=jochen@chromium.org, vogelheim@chromium.org
BUG=chromium:617892

Review-Url: https://codereview.chromium.org/2490783004
Cr-Commit-Position: refs/heads/master@{#40949}