Downloads : Progress infobar controller

This CL adds the controller changes for the download progress infobar.
UX : https://docs.google.com/presentation/d/1wfX6QkEzDbt3Bx2hj6v85_lND1iSUUfXIZN6TMhREuo

Key points :
1 - The controller listens to all the updates from downloads and offline
content agggregator. It converts download items into offline items,
keeps track of all the offline items in a current session.
2 - The state transitions between accelerating, downloading one or more
files, downloads complete/failed/pending etc is handled in the controller as per the UX.

Bug: 1003754
Change-Id: Id7d3d9f91616c79facbcf87a82346f70709e2af2
Reviewed-on: https://chromium-review.googlesource.com/1003873
Reviewed-by: Min Qin <[email protected]>
Reviewed-by: David Trainor <[email protected]>
Commit-Queue: Shakti Sahu <[email protected]>
Cr-Commit-Position: refs/heads/master@{#559349}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 757e4c5..33fadb46 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1223,6 +1223,9 @@
 extern const char kDownloadsLocationChangeName[];
 extern const char kDownloadsLocationChangeDescription[];
 
+extern const char kDownloadProgressInfoBarName[];
+extern const char kDownloadProgressInfoBarDescription[];
+
 extern const char kEnableAndroidPayIntegrationV1Name[];
 extern const char kEnableAndroidPayIntegrationV1Description[];