GitLab

- Add GN build file (BUILD.gn) for content_resources target
(//content/app/resources).
- Add GRD file for grit (content_resources.grd).
- Add GYP file (content_resources.gyp) to generate the grit header file
from content_resources.grd

Not hooked up yet, this will come in the next CL.

BUG=338338
TEST=None
R=avi@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/475223004

Cr-Commit-Position: refs/heads/master@{#290150}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290150 0039d316-1c4b-4281-b951-d872f2087c98