Previously the String constructor and the Symbol.prototype.toString
methods had to compute the descriptive string for a Symbol on the fly,
which can produce a lot of garbage when this happens a lot, i.e. when
the String representation of a Symbol is used often. Now instead of
doing this on-demand we can just do it upfront when creating the Symbol.

That way we also ensure that we won't throw an exception when accessing
the descriptive string of a Symbol, due to potential String length
overflow, but have the exception during Symbol creation upfront, which
is a lot less surprising behavior.

BUG=v8:6278,v8:6344,v8:6350
TBR=mlippautz@chromium.org
R=ishell@chromium.org

Review-Url: https://codereview.chromium.org/2900703002
Cr-Commit-Position: refs/heads/master@{#45479}