DevTools: Replace Breakpoint.primaryUILocation with UISourceCode's projectId, path and lineNumber.

Breakpoint object survives reload (because of the provisional breakpoints) while uiSourceCodes should often be destroyed on any navigation.
This patch makes Breakpoint no longer keep a reference on the uiSourceCode object and query it from workspace when needed.

R=pfeldman

Review URL: https://chromiumcodereview.appspot.com/23704010

git-svn-id: svn://svn.chromium.org/blink/trunk@158105 bbb929c8-8fbe-4397-9dbb-9b2b20218538