Move some content url constants to /url.

This CL moves the content url constants below to /url:
  content::kBlobScheme
  content::kDataScheme
  content::kFileScheme
  content::kFileSystemScheme
  content::kFtpScheme
  content::kJavaScriptScheme
  content::kMailToScheme

This is a follow up to https://codereview.chromium.org/254763005.

BUG=306258
TBR=finnur@chromium.org,mkosiba@chromium.org,derat@chromium.org
TEST=compile

Review URL: https://codereview.chromium.org/273193004

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@273359 4ff67af0-8c30-449e-8e8b-ad334ec8d88c