blink/bindings: Simplify null ParkableString.

Currently, we have:
ParkableString -> ParkableStringImpl -> Null String
This removes the indirections, saving memory and making code simpler.

Bug: 877044
Change-Id: Ib6d22cad836c1eca9918e0198f5e6a1af8fccbe2
Reviewed-on: https://chromium-review.googlesource.com/1213262
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589888}
7 files changed