Rename WebCore namespace to blink

This CL uses a #define to rename the WebCore namespace to blink. The #define is
scoped to only affect files inside the Blink repository. After this CL rolls
into the main waterfall, I'll post CLs that update the source code to use
"blink" instead of "WebCore". Once we've done all the renames, I'll revert this
CL.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/399513002

git-svn-id: svn://svn.chromium.org/blink/trunk@178289 bbb929c8-8fbe-4397-9dbb-9b2b20218538