Run Chrome OS accessibility histograms on main thread.

content/browser/accessibility/browser_accessibility_state_impl.cc

Index: content/browser/accessibility/browser_accessibility_state_impl.cc
diff --git a/content/browser/accessibility/browser_accessibility_state_impl.cc b/content/browser/accessibility/browser_accessibility_state_impl.cc
index 44a8d29d2a54f9e724ef671c7878424f10ff6ada..01ffc2a7913f1c00d1bb06f154f49f46f07e1bfe 100644
--- a/content/browser/accessibility/browser_accessibility_state_impl.cc
+++ b/content/browser/accessibility/browser_accessibility_state_impl.cc
@@ -49,13 +49,24 @@ BrowserAccessibilityStateImpl::BrowserAccessibilityStateImpl()
     accessibility_mode_ = AccessibilityModeComplete;
   }
 
+#if defined(OS_WIN)

[oshima, 2012/12/03 22:09:23]

From the stack trace, the problem seems to be chromeos specific.
Or other platforms (mac/linux) are accessing BrowserProcessImpl from non UI
threads as well?

[dmazzoni, 2012/12/03 23:32:30]

Currently there's no Mac-specific or Linux-specific code. The desire to do some
processing on another thread was motivated by Windows-only needs, so I think it
makes sense for Windows to be the special case - the main thread seems like a
more sensible default for future changes to other platforms.

+  // On Windows, UpdateHistogram calls some system functions with unknown
+  // runtime, so call it on the file thread to ensure there's no jank.
+  // Everything in that method must be safe to call on another thread.
+  BrowserThread::ID browser_thread = BrowserThread::FILE;
+#else
+  // On all other platforms, UpdateHistogram should be called on the main
+  // thread.
+  BrowserThread::ID browser_thread = BrowserThread::UI;

[oshima, 2012/12/03 22:09:23]

the name |browser_thread| doesn't make much sense. Can you use something like
histogram_update_thread?

[dmazzoni, 2012/12/03 23:32:30]

Done.

+#endif
+
   // UpdateHistogram only takes a couple of milliseconds, but run it on
   // the FILE thread to guarantee there's no jank.
   // And we need to AddRef() the leaky singleton so that Bind doesn't
   // delete it prematurely.

[oshima, 2012/12/03 22:09:23]

update this comment too. May be just move it inside OS_WIN?

[dmazzoni, 2012/12/03 23:32:30]

Thanks, meant to do that earlier. Done.

   AddRef();
   BrowserThread::PostDelayedTask(
-      BrowserThread::FILE, FROM_HERE,
+      browser_thread, FROM_HERE,
       base::Bind(&BrowserAccessibilityStateImpl::UpdateHistogram, this),
       base::TimeDelta::FromSeconds(kAccessibilityHistogramDelaySecs));
 }