Add blink_resources target.

This target will help us in the Chromium side to move
webkit_resources.gyp more easily to content.

Currently webkit_resources declares a blink_chromium_resources action
which includes third_party/WebKit/public/blink_resources.grd.

This new "blink_resources" target will be used to remove that. Once this
lands, Chromium will be updated to use this target from Blink.

BUG=338338
TEST=see above
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/390723002

git-svn-id: svn://svn.chromium.org/blink/trunk@178275 bbb929c8-8fbe-4397-9dbb-9b2b20218538