Always formats full GN files that are modified since the format should
be canonical.

I tried to make it consistent in style with the existing formatters. I've
tested running it locally. For "git cl format" it formats GN files. For
"git cl format --diff" it outputs the full formatted files like clang but
doesn't modify. For "git cl format --dry-run" it doesn't modify or output.

TEST=Works running locally.
BUG=

Review URL: https://codereview.chromium.org/1666403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298617 0039d316-1c4b-4281-b951-d872f2087c98