DevTools: Introduce DevToolsTargetImpl to unify debug target across several places in Chrome

R=vsevik@chromium.org
TBR=jochen@chromium.org for chrome.gyp
BUG=None

Review URL: https://codereview.chromium.org/27217005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230720 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230720