Downloads: replace BrowserThread::FILE with task scheduler.

content/common/quarantine/quarantine_win.cc

Index: content/common/quarantine/quarantine_win.cc
diff --git a/content/common/quarantine/quarantine_win.cc b/content/common/quarantine/quarantine_win.cc
index 3feb1966af8854527e17aa2f195465ce6cdcd720..1c5d0fc9b24f1c164d12250c29c32060fa7a30d1 100644
--- a/content/common/quarantine/quarantine_win.cc
+++ b/content/common/quarantine/quarantine_win.cc
@@ -25,6 +25,7 @@
 #include "base/strings/string_split.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread_restrictions.h"
+#include "base/win/scoped_com_initializer.h"
 #include "base/win/scoped_comptr.h"
 #include "base/win/scoped_handle.h"
 #include "url/gurl.h"
@@ -302,6 +303,10 @@ QuarantineFileResult QuarantineFile(const base::FilePath& file,
                                     const std::string& client_guid) {
   base::ThreadRestrictions::AssertIOAllowed();
 
+  // TODO(siggi): As this is a rare operation, this seems better than putting

[Sigurður Ásgeirsson, 2017/07/20 12:36:34]

Gab: there are two places where downloading needs COM on windows, the other one
is in BaseFile::MoveFileAndAdjustPermissions. I think ideally these would be
invoked on a common/global COM STA thread, but I feel that's outside the scope
of this change. WDYT?

[robliao, 2017/07/20 18:29:17]

I was actually thinking about making ScopedCOMInitializer DCHECK for non COM STA
Task Runners. 

Is it difficult to move this call to a COM STA Task Runner?

[Sigurður Ásgeirsson, 2017/07/20 19:48:15]

I'm not sure, but I'm pretty sure it's out of scope for this CL, which is
already a 6 week drag-on :). I'm more worried that there are untested cases in
downloads that stray into COM or the shell, that I haven't found.

The safe thing to do is to drop downloads on the COM STA sequence on Windows?

[robliao, 2017/07/20 20:15:53]

Dropping all of downloads in a COM STA sounds fine to me.

+  //     all download activity on a COM STA sequence.
+  base::win::ScopedCOMInitializer com_initializer;
+
   int64_t file_size = 0;
   if (!base::PathExists(file) || !base::GetFileSize(file, &file_size))
     return QuarantineFileResult::FILE_MISSING;