Allow disabling Aura's pull-to-refresh for touchpad

This CL changes --pull-to-refresh flag to have 4 possible values:
 - "Default": Currently the same as Disabled;
 - "Disabled": No PTR;
 - "Enabled": PTR enabled for both touchpad and touchscreen;
 - "Enabled for touchscreen only": PTR enabled only for touchscreen.

BUG=702404
TEST=OverscrollControllerTest.PullToRefresh* in content_unittest

Change-Id: I18e1e39f967e1041f56f25b49e42b9a6dfc934fa
Reviewed-on: https://chromium-review.googlesource.com/998549
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Sadrul Chowdhury <[email protected]>
Commit-Queue: Mohsen Izadi <[email protected]>
Cr-Commit-Position: refs/heads/master@{#550288}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 6fc40e11..0744b0f 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -764,6 +764,7 @@
 
 extern const char kPullToRefreshName[];
 extern const char kPullToRefreshDescription[];
+extern const char kPullToRefreshEnabledTouchscreen[];
 
 extern const char kPushApiBackgroundModeName[];
 extern const char kPushApiBackgroundModeDescription[];