This removes:
- CodeBreakIterator for FCG code.
- RelocModes for debug breaks.
- Code generator for debug break slots.
- GC support for debug break slots.
- Code flag to indicate code with debug break slots.
- Builtin type DBG.
- Mechanisms to replace FCG code in the debugger and LiveEdit.
- Runtime entry to the debugger from debug break slots.

R=bmeurer@chromium.org, rmcilroy@chromium.org, ulan@chromium.org

Bug: v8:6409
Change-Id: I5662c8800e3ef1b1584ad107bfe0aae26c9d8abb
Reviewed-on: https://chromium-review.googlesource.com/613263Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47364}