Split devtools.gyp into a gyp and a gypi file.

The devtools.gyp file is referenced from outside of blink and should
therefore be moved to blink/public

BUG=none
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/100503004

git-svn-id: svn://svn.chromium.org/blink/trunk@163251 bbb929c8-8fbe-4397-9dbb-9b2b20218538