Move the user agent styles sheets to blink_resources.grd (Part 2) with Unit test...

Move the user agent styles sheets to blink_resources.grd (Part 2) with Unit test support

Blink should make use of blink_resources.grd for the inline resources of user agent stylesheets.
This removes the dependency upon using make-file-arrays.py which embeds these resources as strings.

This patch adds the necessary plumbing required for the webkit_unit_tests target.
As the resources are being moved into blink_resources.grd, this generates blink_resources.pak.
This pak doesn't exists as a separate pak file but its repacked into "content_shell.pak".
The same must be loaded into the data_pack_ of the TestContentClient so that the webkit (blink) unit
tests run successfully.

Part 1: https://codereview.chromium.org/436843004/
Part 1 with conditional: https://codereview.chromium.org/440683004/
Part 2: https://codereview.chromium.org/422023008
Part 3: https://codereview.chromium.org/438843004/

BUG=312586

Review URL: https://codereview.chromium.org/456413002

Cr-Commit-Position: refs/heads/master@{#294554}