Move CHA field back to Isolate, pending a permanent fix.

Move CHA field back to Isolate, pending a permanent fix.

Since the Thread structure is currently cleared on Enter/ExitIsolate,
there is a risk that CHA is cleared if an embedder callback executes during
compilation and switches/spawns isolates. This CL eliminates that risk.

However, Isolate is still not a good long-term place for CHA,
since it doesn't fit with a concurrent compilation model. Issue 23153 tracks
the work towards a permanent solution that couples its life cycle with
compilation.

R=fschneider@google.com, iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45121

[koda, 2015-04-09 20:26:52]

Please note that, if possible, I'd like to get this into tomorrow's full push
for 1.10.

[Ivan Posva, 2015-04-10 07:04:07]

LGTM -ip

[Florian Schneider, 2015-04-10 09:09:01]

fschneider@google.com changed reviewers:
+ fschneider@google.com

[Florian Schneider, 2015-04-10 09:09:02]

lgtm

[koda, 2015-04-13 21:02:02]

Committed patchset #2 (id:20001) manually as r45121 (presubmit successful).