blink: Add support for locking ParkableString.

ExternalStringResource can be locked in V8, meaning that it should not
be parked. Add support in Chrome for this.
Separately, some blink strings will never be parked. For these, allow V8 to
cache the data pointer.

Bug: 877044
Change-Id: I0100a3b13887c417dfcf568186c9cf7f9c57706f
Reviewed-on: https://chromium-review.googlesource.com/1203376
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589176}
5 files changed