unbundle: add replace_gn_files.py and unbundled GN file for libxml

build/linux/unbundle/README

 This directory contains files that make it possible to use system libraries.
 
 For more info please read the following:
 
  - https://fedoraproject.org/wiki/Packaging:No_Bundled_Libraries
  - https://wiki.gentoo.org/wiki/Why_not_bundle_dependencies
  - http://www.debian.org/doc/debian-policy/ch-source.html#s-embeddedfiles
 
 For more Chromium-specific context please read
 http://spot.livejournal.com/312320.html .
 
+Additional resources which might provide even more context:
+
+  - http://events.linuxfoundation.org/sites/events/files/slides/LinuxCon%202014%20Slides_0.pdf
+  - https://lwn.net/Articles/619158/
+
 This directory is provided in the source tree to follow above guidelines.
 It is a compromise solution which takes into account Chromium developers
-who want to avoid the perceived burden of more conditional code in gyp,
+who want to avoid the perceived burden of more conditional code in build files,
 and expectations of Open Source community, where using system-provided
 libraries is the norm.
 
 Usage:
 
 1. remove_bundled_libraries.py <preserved-directories>
 
    For example: remove_bundled_libraries.py third_party/mesa
 
    The script scans sources looking for third_party directories.
    Everything that is not explicitly preserved is removed (except for
    gyp files), and the script fails if any directory passed on command
    line does not exist (to ensure list is kept up to date).
 
    This is intended to be used on sources extracted from a tarball,
    not a repository.
 
    NOTE: by default this will not remove anything (for safety). Pass
    --do-remove flag to actually remove files.
 
-2. replace_gyp_files.py <gyp-flags>
+2. Pick the script to run depending on whether you use GYP or GN:
 
-   For example: replace_gyp_files.py -Duse_system_harfbuzz=1
+2a. replace_gyp_files.py <gyp-flags>
 
-   The script ignores flags other than -D for convenience. This makes it
-   possible to have a variable e.g. ${myconf} with all the options, and
-   execute:
+    For example: replace_gyp_files.py -Duse_system_harfbuzz=1
 
-   build/linux/unbundle/replace_gyp_files.py ${myconf}
-   build/gyp_chromium ${myconf}
+    The script ignores flags other than -D for convenience. This makes it
+    possible to have a variable e.g. ${myconf} with all the options, and
+    execute:
+
+    build/linux/unbundle/replace_gyp_files.py ${myconf}
+    build/gyp_chromium ${myconf}
+
+2b. replace_gn_files.py --system-libraries lib...
+
+    For example: replace_gn_files.py --system-libraries libxml