Fix BrowserAccessibilityManagerMac: set empty selected text range.

Raises an NSInvalidArgumentException if call setObject for NSMutableDictionary
with a nil value. Dictionary uses NSNull instead of nil.
Selected text marker range may be nil sometimes
(see BrowserAccessibilityCocoa::selectedTextMarkerRange), that leads to a crashe
in BrowserAccessibilityManagerMac::GetUserInfoForSelectedTextChangedNotification().

R=aboxhall@chromium.org, dmazzoni@chromium.org, dtseng@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2161623002
Cr-Commit-Position: refs/heads/master@{#407194}

content/browser/accessibility/browser_accessibility_manager_mac.mm

@@ -425,10 +425,14 @@ NSDictionary* BrowserAccessibilityManagerMac::
     focus_object = focus_object->GetClosestPlatformObject();
     auto native_focus_object = ToBrowserAccessibilityCocoa(focus_object);
     if (native_focus_object && [native_focus_object instanceActive]) {
-      [user_info setObject:[native_focus_object selectedTextMarkerRange]
-                    forKey:NSAccessibilitySelectedTextMarkerRangeAttribute];
       [user_info setObject:native_focus_object
                     forKey:NSAccessibilityTextChangeElement];
+
+      id selected_text = [native_focus_object selectedTextMarkerRange];
+      if (selected_text) {
+        [user_info setObject:selected_text
+                      forKey:NSAccessibilitySelectedTextMarkerRangeAttribute];
+      }
     }
   }