[blink/bindings] CHECK() reference counting in ParkableString.

Lock() / Unlock() pairs must be balanced, and should not under/overflow
the lock_depth_ counter. Make sure this is CHECK()ed everywhere.

Change-Id: I041370afb282c62cc2cfa2e62e55306de4e0d69c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2734815


Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859734}