Revert 123140 - Use SequencedWorkerPool for disk operations in TraceSubscriberStdio.

Revert 123140 - Use SequencedWorkerPool for disk operations in TraceSubscriberStdio.

Reverted since it causes compile failure on Win/Aura. 
(scoped_refptr of TraceSubscriberStdioImpl which is only forward-declared)

This class was hitting a ThreadRestrictions assert because it called
OpenFile on the UI thread. To reduce unnecessary copying, I've changed
the OnTraceDataCollected argument from std::string to RefCountedString.

BUG=None
TEST=content_unittests

Review URL: http://codereview.chromium.org/9333003

TBR=husky@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=123146