Improve error messaging of webRequest API in case of conflicts


BUG=126438
TBR=jochen@chromium.org,estade@chromium.org



Review URL: https://chromiumcodereview.appspot.com/10407105

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167673 0039d316-1c4b-4281-b951-d872f2087c98
27 files changed