libxslt: Update License File's location.

libxslt's source code was moved to src/ in commit 6f776b2a ("Check in
the libxslt roll script"), but README.chromium wasn't updated to point to
the license file's new location.

Bug: 708393
Change-Id: I041569545e3c9d536b826bfaa198605ba5ca6a88
Reviewed-on: https://chromium-review.googlesource.com/597848
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491388}

third_party/libxslt/README.chromium

@@ -3,7 +3,7 @@ URL: http://xmlsoft.org/XSLT
 Version: ac341cbd792ee572941cc9a66e73800219a1a386
 Security Critical: yes
 License: MIT
-License File: Copyright
+License File: src/Copyright
 
 Description: