The algorithm used to compute the textual differences uses requires
quadratic space (in the size of the input scripts). Previously the
implementation was naively allocating a single matrix, which is commonly
very sparse, since the expectation for LiveEdit is that only a small
portion of the script is actually altered. So we can use a std::map here
instead to reduce the cost.

We can also significantly reduce the cost (especially of the stack grow
due to the recursion) by precomputing the common prefix, and pre-filling
the table for the common suffix, both of which are also assumed to make
up for the majority of the script in case of LiveEdit.

This is still only ducktape, but should mitigate the crashes in the wild
significantly. Ideally we'd eventually replace this with an
implementation of the Myers algorithm that runs in linear space.

Fixed: chromium:1199807
Change-Id: Ib5fa0b1aa63c67631f919dc3b6641dfc0b20ae74
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2867470Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74344}