Add the download component and initial setup

chrome/common/chrome_constants.cc

Index: chrome/common/chrome_constants.cc
diff --git a/chrome/common/chrome_constants.cc b/chrome/common/chrome_constants.cc
index 466f94d3b8f816a61178a9744d9c821857202fbd..7ce201367106f6bbb0c357f49683d7d0c9edfc72 100644
--- a/chrome/common/chrome_constants.cc
+++ b/chrome/common/chrome_constants.cc
@@ -140,6 +140,8 @@ const base::FilePath::CharType kCRLSetFilename[] =
     FPL("Certificate Revocation Lists");
 const base::FilePath::CharType kCustomDictionaryFileName[] =
     FPL("Custom Dictionary.txt");
+const base::FilePath::CharType kDownloadServicePersistedStorageDirname[] =
+    FPL("Download Service/persisted");

[xingliu, 2017/05/01 23:56:54]

nit: "/" This might not work on windows. Not sure, but all path with a separator
here seems to be platform specified.

[Peter Beverloo, 2017/05/02 16:12:20]

+1. Best to just give the "Download Service" directory to the Service and have
it append "persisted" (and/or "temporary") itself?

[David Trainor- moved to gerrit, 2017/05/03 06:02:58]

Can do!  I was looking at Offline Page below which all had a '/'.

 const base::FilePath::CharType kExtensionActivityLogFilename[] =
     FPL("Extension Activity");
 const base::FilePath::CharType kExtensionsCookieFilename[] =