Restore previous input device to display pairing behavior.

We want to always associate a input device so that we don't accidentally break
anyone who is depending on this behavior.

BUG=582516

patch from issue 1652513002 at patchset 20001 (http://crrev.com/1652513002#ps20001)

(cherry picked from commit d5c1e3b0b0a7b5b960ef16062d5722f24a1ea698)

Review URL: https://codereview.chromium.org/1659543002 .

Cr-Commit-Position: refs/branch-heads/2623@{#216}
Cr-Branched-From: 92d7753-refs/heads/master@{#369907}

ash/touch/touchscreen_util.cc

@@ -260,6 +260,18 @@ void AssociateTouchscreens(
     VLOG(1) << "Unassociated display " << display->name();
   for (const ui::TouchscreenDevice* device : devices)
     VLOG(1) << "Unassociated device " << device->name;
+
+  // Match any unassociated devices to a display. This maintains existing
+  // pairing behavior. See crbug.com/582516.
+  if (displays.size() > 0 && devices.size() > 0) {
+    DisplayInfo* display = displays[0];
+
+    for (const ui::TouchscreenDevice* device : devices) {
+      VLOG(1) << "Randomly matching device " << device->name << " to display "
+              << display->name();
+      Associate(display, device);
+    }
+  }
 }
 
 }  // namespace ash