Lots of web pages have really frequently firing timers that keep the
profiler thread spinning if we require a period of JS inactivity
before suspending the profiler. While it's possible to throttle it by
increasing the sleep delay and adjusting the duration of the required
inactive period, it seemed much simpler to just stop it immediately on
exiting JS.

Stopping the profiler this way effectively turned off two optimization
heuristics: 1) eager optimization (it's reset on waking up the
profiler and now the profiler wakes up much more frequently) and 2)
optimization throttling based on JS to non-JS state ratio (the ratio
is now 100%). I removed these two heuristics and found no performance
regressions so far.

R=ager@chromium.org
BUG=crbug.com/77625
TEST=none

Review URL: http://codereview.chromium.org/7274024

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8472 ce2b1a6d-e550-0410-aec6-3dcde31c8c00