Add skeleton code for ServiceWorkerURLRequestJob

No meaningful code yet, mostly just for placeholders & very basic plumbing.

NOTE: A lot of details are omitted.

BUG=349319

Review URL: https://codereview.chromium.org/185133004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255005 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255005
ui target does not need zlib and libpng to build.

ui target should link fine without those deps. This is because zlib and
libpng are only used by ui/gfx/.

BUG=299841
TEST=ui_unittests, gfx_unittests
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/180833005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255006 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255006