Remove global #define WebKit blink

Now that we've updated all the code from the WebKit namespace to the blink
namespace, we can remove this global #define.

TBR=jamesr@chromium.org
BUG=295096

Review URL: https://codereview.chromium.org/63823003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233627 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233627