Change GetFontListAsync() to use the blocking pool.

PPB_Font_Dev.GetFontFamilies() is a sync operation that waits for GetFontListAsync() to finish on the FILE thread in the browser process.
If the thread is busy, it could hang for quite a while.
Some Pepper Flash hangs are caused by this.

BUG=155931
TEST=None


Review URL: https://chromiumcodereview.appspot.com/11143028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@162383 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/font_list_async.cc

@@ -20,7 +20,7 @@ void ReturnFontListToOriginalThread(
   callback.Run(result.Pass());
 }
 
-void GetFontListOnFileThread(
+void GetFontListInBlockingPool(
     BrowserThread::ID calling_thread_id,
     const base::Callback<void(scoped_ptr<base::ListValue>)>& callback) {
   scoped_ptr<base::ListValue> result(GetFontList_SlowBlocking());
@@ -38,8 +38,8 @@ void GetFontListAsync(
   DCHECK(well_known_thread)
       << "Can only call GetFontList from a well-known thread.";
 
-  BrowserThread::PostTask(BrowserThread::FILE, FROM_HERE,
-      base::Bind(&GetFontListOnFileThread, id, callback));
+  BrowserThread::PostBlockingPoolTask(FROM_HERE,
+      base::Bind(&GetFontListInBlockingPool, id, callback));
 }
 
 }  // namespace content