Check in the libxslt roll script.

This rolls libxslt to the same upstream point,
ac341cbd792ee572941cc9a66e73800219a1a386, but with some important
differences:

- The script which does the roll is included along with the roll
  itself.

- The source is consumed similarly to libxml, and similarly to how
  other projects integrate libxslt, by first producing a distribution
  which is then unpacked into the Chromium tree under libxslt/src.

- A patches is applied to the source and it is easy to add more
  patches; the patches will live in the source tree.

This also updates README.chromium to omit mentioning two patches we
already rolled in from upstream: crbug.com/676623 which was fixed
upstream in
https://git.gnome.org/browse/libxslt/commit/?id=08ab2774b870de1c7b5a48693df75e8154addae5
and crbug.com/583171 which was fixed upstream after
https://git.gnome.org/browse/libxslt/commit/?id=91d0540ac9beaa86719a05b749219a69baa0dd8d
per crbug.com/583171#c22

BUG=708393

Review-Url: https://codereview.chromium.org/2865973002
Cr-Commit-Position: refs/heads/master@{#470285}
132 files changed