DevTools: Get rid of BreakpointManager._breakpoints Map.

BreakpointManager._breakpoints map semantics is unclear and its contents essentially duplicates the one of
BrekapointManager._breakpointsforUISourceCode. Removing it is another step torwards simplifying breakpoint manager.

R=pfeldman

Review URL: https://codereview.chromium.org/23771008

git-svn-id: svn://svn.chromium.org/blink/trunk@158741 bbb929c8-8fbe-4397-9dbb-9b2b20218538