Move the user agent styles sheets to blink_resources.grd (Part 1) with conditional includes

This is continuation to earlier CL [1] with the conditional statements being added for
specific platform CSS file includes.

Part 1: https://codereview.chromium.org/436843004/
Part 2: https://codereview.chromium.org/422023008/
Part 3: https://codereview.chromium.org/438843004/

BUG=312586
NOTRY=true
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/440683004

git-svn-id: svn://svn.chromium.org/blink/trunk@179504 bbb929c8-8fbe-4397-9dbb-9b2b20218538