Before serializing an external string for a native source, we replace
its resource field with the type and index of the native source. Upon
deserialization, we restore the resource.

This change also removes the native source caches with a more straight-
forward mechanism to find the resource type and index.

R=ulan@chromium.org

Review-Url: https://codereview.chromium.org/2807023003
Cr-Commit-Position: refs/heads/master@{#44545}