Adds a pref and a policy to decide if PDFs should always be opened externally.

chrome/browser/download/download_prefs.h

Index: chrome/browser/download/download_prefs.h
diff --git a/chrome/browser/download/download_prefs.h b/chrome/browser/download/download_prefs.h
index 01bd1c9d5ec2e70b849a01f937b87a6424738eef..2823f0e79161431e868cf7ce8d3e6bbfcdfc4a92 100644
--- a/chrome/browser/download/download_prefs.h
+++ b/chrome/browser/download/download_prefs.h
@@ -8,6 +8,7 @@
 #include <set>
 
 #include "base/files/file_path.h"
+#include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "build/build_config.h"
 #include "components/prefs/pref_member.h"
@@ -87,8 +88,12 @@ class DownloadPrefs {
   // Return whether the user prefers to open PDF downloads in the platform's
   // default reader.
   bool ShouldOpenPdfInSystemReader() const;
+
+  // Used by tests to disable version checks for Adobe.
+  void DisableAdobeVersionCheckForTests();
 #endif
 
+
   void ResetAutoOpen();
 
  private:
@@ -112,6 +117,7 @@ class DownloadPrefs {
 
 #if defined(OS_WIN) || defined(OS_LINUX) || defined(OS_MACOSX)
   bool should_open_pdf_in_system_reader_;
+  bool disable_adobe_version_check_for_tests_;

[asanka, 2016/09/30 14:34:53]

Rather than disable the version check, can we force the test to succeed? I.e.
Make adobe_reader_info_win.h: IsAdobeReaderUptodate() and
IsAdobeReaderDefaultPDFViewer() overridable for testing?

This way the effects of the Adobe reader version check will be uniform rather
than just being overridden here. 

[pastarmovj, 2016/10/04 10:06:34]

I agree with your comment but for the sake of expediting this CL into the
current version and due to lack of other direct use cases of this I would like
to make this work a separate issue and filed crbug/652624 are you ok with that?

 #endif
 
   DISALLOW_COPY_AND_ASSIGN(DownloadPrefs);