Reland "Prevent liveedit on or under generators with open activations"

The change relative to the previous CL is a logic change in
DropActivationsInActiveThreadImpl.  The previous CL skipped the matcher
unless the frame was a JS frame; this was correct for
MultipleFunctionTarget but not for SingleFrameTarget.

I have not been able to reproduce the original failures on either
architecture (ia32 or x64; stack frame dropping is unsupported on other
architectures).

R=yangguo@chromium.org
LOG=N
TEST=mjsunit/harmony/generators-debug-liveedit.js
BUG=

Review URL: https://codereview.chromium.org/270283002

Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21419 ce2b1a6d-e550-0410-aec6-3dcde31c8c00