Adds support for zone allocated (off-heap) ConsumedPreParsingScopeData to
enable worker-thread access to PreParsingScopeData during parallel IIFE
compile tasks.

In order to avoid code-duplication, a templated
BaseConsumedPreParsingScopeData is added which implements the logic for
decoding the bytestream into scope data. Two implementations of this
base class are instantiated for each of the underlying serialized scope date:
  - ZoneConsumedPreParsedScopeData for exposing ZonePreParsedScopeData
  - OnHeapConsumedPreParsedScopeData for exposing on-heap PreParsedScopeData
The interface for each of these classes is the ConsumedPreParsingScopeData,
which exposes the methods required by the parser to deserialize the required
data.

As a side-cleanup, moved Ucs2CharLength and Utf8LengthHelper implementations
to cc file so that we don't get a linker error if one of them are unused by
the cc file including the header.


BUG=v8:8041

Change-Id: Id502312d32fe4a9ddb6f5d2d9d3e3a9d30b9b27d
Reviewed-on: https://chromium-review.googlesource.com/1199462
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55711}