DebugBreak bytecode fetches current return value from debugger prior
dispatching original handler. So we can change its value on break.

R=leszeks@chromium.org,rmcilroy@chromium.org

Bug: chromium:656150
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I82d0bc82ff49923a748c0084d252d0fd214a2db8
Reviewed-on: https://chromium-review.googlesource.com/731679Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49122}