[DevTools] Move dom breakpoints to DOMDebuggerModel

DOMBreakpointsSidebarPane just renders dom breakpoints now. Removed
pane's proxy and moved context menu into it.

The pane should not be referenced outside of components anymore.

BUG=none

Review-Url: https://codereview.chromium.org/2837203002
Cr-Commit-Position: refs/heads/master@{#467121}
10 files changed