Crankshaft just unconditionally deoptimizes the code when the length of
a string addition result would overflow. In order to protect against
deopt loops we insert a global protector cell.

We will use the same mechanism for inlining certain string additions
into TurboFan as well, and protecting against overflow (we will also
extend this to deal with String.prototype.concat and friends once we
get there).

BUG=v8:5404
R=jarin@chromium.org,hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2348293002
Cr-Commit-Position: refs/heads/master@{#39511}