Fix memory leak in TraceSubscriberStdioImpl.

Fix memory leak in TraceSubscriberStdioImpl.

This class uses TraceBuffer internally, via a callback object.
The callback references the TraceSubscriberStdioImpl so there
was a reference cycle. Fixed by explicitly clearing the callback.

BUG=115411
TEST=TraceSubscriberStdioTest under Valgrind


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=147482

[Reid Kleckner (google), 2012-07-18 17:46:34]

LGTM

[commit-bot: I haz the power, 2012-07-19 12:36:41]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/husky@chromium.org/10797012/1

[commit-bot: I haz the power, 2012-07-19 12:36:43]

Presubmit check for 10797012-1 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit ERRORS **
Missing LGTM from an OWNER for files in these directories:
    content/browser

[Iain Merrick, 2012-07-19 13:49:17]

Adding sky for OWNERS review.

[sky, 2012-07-19 16:33:33]

LGTM

[commit-bot: I haz the power, 2012-07-19 16:42:10]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/husky@chromium.org/10797012/1

[commit-bot: I haz the power, 2012-07-19 18:02:07]

Change committed as 147482