Introduce git merge driver for the blink reformatting

Introduce git merge driver for the blink reformatting

- Introduce a script (clang_format_merge_driver) that can be used
  as a custom merge driver (see "Defining a custom merge driver"
  section of https://git-scm.com/docs/gitattributes)
- Automatically create a merge driver section in the .git/config of
  the main project, as follows:
  [merge "clang_format_merge_driver"]
     name = clang-format merge driver (http://crbug.com/574611)
     driver = clang_format_merge_driver %O %A %B %L %P
     recursive = binary

Note that this change, as is, does not have any impact (other than
tweaking the git config of the main project). This custom merge driver
is not used until somebody edits a .gitattributes file adding a line
like:
*.cpp           merge=clang_format_merge_driver

BUG=574611