DevTools: Make UISourceCode Target-agnostic

The presence of Target-related mappings in UISC is a severe
dependency violation.
This patch moves those mappings into DebuggerWorkspaceBinding.

R=pfeldman, sergeyv, vsevik

Review URL: https://codereview.chromium.org/464963002

git-svn-id: svn://svn.chromium.org/blink/trunk@180159 bbb929c8-8fbe-4397-9dbb-9b2b20218538