Flatten the Arrays returned and consumed by the v8::Map API

Flatten the Arrays returned and consumed by the v8::Map API

This will significantly simplify the serialization code, as well
as speeding it up (by triggering only a single allocation instead of O(size)
allocations).

BUG=chromium:478263
LOG=y

Review URL: https://codereview.chromium.org/1157843006

Cr-Commit-Position: refs/heads/master@{#28793}