This CL adds support for the following virtual string types:
 - SCRIPT_SOURCE_EXTERNAL_ONE_BYTE_TYPE
 - SCRIPT_SOURCE_EXTERNAL_TWO_BYTE_TYPE
 - SCRIPT_SOURCE_NON_EXTERNAL_ONE_BYTE_TYPE
 - SCRIPT_SOURCE_NON_EXTERNAL_TWO_BYTE_TYPE
 - STRING_EXTERNAL_RESOURCE_ONE_BYTE_TYPE
 - STRING_EXTERNAL_RESOURCE_TWO_BYTE_TYPE

Change-Id: I8d278ede356bae4ba63c1dae45a347f1261c75cc
Reviewed-on: https://chromium-review.googlesource.com/1174392Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55230}