Use FILE thread for disk operations in TraceSubscriberStdio

content/browser/trace_subscriber_stdio.cc

Index: content/browser/trace_subscriber_stdio.cc
===================================================================
--- content/browser/trace_subscriber_stdio.cc	(revision 120564)
+++ content/browser/trace_subscriber_stdio.cc	(working copy)
@@ -21,6 +21,10 @@
 bool TraceSubscriberStdio::OpenFile(const FilePath& path) {
   LOG(INFO) << "Logging performance trace to file: " << path.value();
   CloseFile();
+
+  // We're in the UI thread, so creating a file is not a great thing to do!
+  // But TraceEvent is not an end user feature, so let's disable the check.
+  base::ThreadRestrictions::ScopedAllowIO scoped_allow_io;

[jam, 2012/02/06 20:25:29]

the goal is to eventually remove ThreadRestrictions. adding more instances moves
that work to someone else.

[joth, 2012/02/07 09:14:59]

I understand what you're saying here, but several established uses of it are
commented with words to the effect "this is by design" so I don't think in
reality it's ever going to be removed? e.g.:

http://code.google.com/codesearch#OAMlx_jo-ck/src/base/process_util_linux.cc&...
http://code.google.com/codesearch#OAMlx_jo-ck/src/base/shared_memory_posix.cc...
http://code.google.com/codesearch#OAMlx_jo-ck/src/chrome/browser/rlz/rlz.cc&e...

[jam, 2012/02/17 23:56:34]

If we keep it because a few places are valid, then it'll get abused much more.

I prefer that you don't add this, and instead do this on another thread.
Eventually we want to remove all these, and this just adds work. But I won't let
it block this commit.

[Iain Merrick, 2012/02/17 23:59:33]

Removed in latest patch.

   file_ = file_util::OpenFile(path, "w+");
   if (IsValid()) {
     trace_buffer_.SetOutputCallback(base::Bind(&TraceSubscriberStdio::Write,